Low grade inflammation, as evidenced by basal high sensitivity CRP, is not correlated to outcome measures in IVF

Association between the systemic immune-inflammation index and GnRH antagonist protocol IVF outcomes: a cohort study

RESEARCH QUESTION

What is the relationship between the systemic immune-inflammation index (SII) and IVF outcomes in women undergoing a gonadotrophin-releasing hormone (GnRH) antagonist protocol?

DESIGN

This retrospective cohort study analysed clinical data and blood samples collected before oocyte retrieval from participants undergoing IVF with the GnRH antagonist protocol. Logistic regression and generalized additive models were used to examine the association between SII quartiles and continuous SII values and IVF outcomes.

RESULTS

Higher SII values correlated negatively with biochemical pregnancy, clinical pregnancy, live birth and implantation rates, and positively with early pregnancy loss, independent of age, body mass index, anti-Müllerian hormone and stimulation parameters. The most significant adverse outcomes were observed in the highest SII quartile. A non-linear relationship was identified between log-transformed SII and IVF outcomes, with an inflection point at an SII of approximately 6.72, indicating a threshold effect.

CONCLUSIONS

Elevated SII is associated with poorer IVF outcomes in women after the GnRH antagonist protocol, suggesting its potential as a predictive marker in IVF treatments. Further research is needed to confirm these findings and explore the underlying mechanisms.

High estradiol levels during a long agonist IVF protocol are associated with decreased food intake, higher leptin concentrations, and lower levels of high-sensitivity C-reactive protein

PURPOSE

To study whether different hormonal phases affect appetite regulation, food intake, and concentrations of leptin, glucagon-like peptide-1 (GLP-1), and high-sensitivity C-reactive protein (hs-CRP) during a long agonist in vitro fertilization (IVF) protocol.

METHODS

Fifty-four infertile women were encountered thrice, the first of which was at the beginning of their period (low estradiol). The other two visits were during a gonadotrophin-releasing hormone (GnRH) analog downregulation (low estradiol) and at the end of a follicle-stimulating hormone (FSH) stimulation (high estradiol). The first visit was the reference; the women served as their controls. The concentrations of leptin, GLP-1, and hs-CRP were assessed from plasma. Dietary intake was assessed using food records (FRs). In addition, weight, height, body mass index (BMI), and plasma levels of estradiol, glucose, HbA1c, insulin, and lipids were monitored. Twenty-six of the subjects also had a postprandial test.

RESULTS

During the stimulation protocol, leptin concentrations elevated (P < 0.001), and energy intake decreased (P = 0.03), while estradiol levels increased (P < 0.001). GLP-1 levels unchanged (P = 0.75) and hs-CRP (P = 0.03) concentrations diminished, while estradiol levels increased.

CONCLUSION

No increased food intake or weight gain occurred during the stimulation protocol; thus, leptin may protect from overeating during high estradiol levels, and leptin resistance may not occur during a short follow-up. Also, a favorable anti-inflammatory effect was detected. During this study, we observed no harmful metabolic effects, which might affect negatively maternal health.

Role of Inflammaging on the Reproductive Function and Pregnancy

During female lifetime and pregnancy, inflammation and cellular senescence are implicated in physiological processes, from ovulation and menstruation, to placental homeostasis and delivery. Several lifestyles, nutritional, and environmental insults, as well as long-lasting pregestational inflammatory diseases may lead to detrimental effects in promoting and sustaining a chronic excessive inflammatory response and inflammaging, which finally contribute to the decay of fertility and pregnancy outcome, with a negative effect on placental function, fetal development, and future health risk profile in the offspring. Maladaptation to pregnancy and obstetric disease may in turn increase maternal inflammaging in a feedback loop, speeding up aging processes and outbreak of chronic diseases. Maternal inflammaging may also impact, through transgenerational effects, on future adult health. Hence, efficacious interventions should be implemented by physicians and healthcare professionals involved in prevention activities to reduce the modifiable factors contributing to the inflammaging process in order to improve public health.

Low-grade inflammation is negatively associated with live birth in women undergoing IVF

RESEARCH QUESTION

Is low-grade inflammation, detected by C-reactive protein (CRP), a marker of IVF outcome addressing both blastocyst quality and pregnancy outcome?

DESIGN

This sub-study of a multicentre randomized controlled trial included 440 women undergoing IVF treatment with a gonadotrophin-releasing hormone (GnRH) antagonist protocol. Serum CRP was measured on cycle day 2-3 (baseline) and on the day of ovulation triggering. The association between CRP concentrations and reproductive outcomes (number of retrieved oocytes, number of good-quality blastocysts, pregnancy, pregnancy loss and live birth), were analysed, adjusting for relevant confounders.

RESULTS

A negative association was found between higher baseline CRP concentrations and live birth rate (odds ratio [OR] 0.77, 95% confidence interval [CI] 0.62-0.96, P = 0.02) and higher CRP concentrations at baseline were associated with pregnancy loss among women who conceived (OR 1.37, 95% CI 1.07-1.76, P = 0.01). When testing for a specific cut-off, CRP concentrations above 2.34 (the highest quartile) were more likely to be associated with pregnancy loss (P = 0.02) and a lower chance of live birth (P = 0.04) compared with the lowest quartile. No associations were found between CRP concentrations and pregnancy outcomes on the day of ovulation triggering, and there were no associations between CRP concentrations and the number of good-quality blastocysts.

CONCLUSIONS

Higher CRP concentrations at cycle day 2-3, before starting ovarian stimulation, are negatively associated with chance of live birth, possibly because of an increased risk of pregnancy loss. No association was found between the number of good-quality blastocysts and CRP concentration. More studies are needed to investigate the impact of low-grade inflammation.

Serum C-reactive protein levels are associated with clinical pregnancy rate after in vitro fertilization among normal-weight women

OBJECTIVE

To assess whether low-grade inflammation, measured by serum high-sensitivity C-reactive protein (hsCRP) levels, is associated with in vitro fertilization (IVF) outcomes.

DESIGN

A retrospective study.

SETTING

University-affiliated IVF center.

PATIENTS

In the present study, 875 women of normal weight who underwent their first fresh embryo transfer (ET) cycles for IVF treatment were divided into three groups according to serum concentrations of hsCRP.

INTERVENTIONS

Serum from women undergoing IVF was collected on days 2-4 of a spontaneous menstrual cycle prior to the commencement of ovarian stimulation.

MAIN OUTCOME MEASURES

The IVF outcomes included implantation, biochemical pregnancy, clinical pregnancy, miscarriage and live birth rates.

RESULTS

The women were divided into three groups according to the baseline serum levels of hsCRP as follows: low hsCRP (<1 mg/L; n=517), medium hsCRP (1-3 mg/L; n= 270), high hsCRP (>3 mg/L; n=88). The maternal age was similar among the three groups. The women in the high and medium hsCRP group had significantly higher BMI compared with those in the low hsCRP group. The protocol of controlled ovarian hyperstimulation, the gonadotropin dose administered, the serum estradiol levels, progesterone levels and the endometrial thickness on the day of triggering, as well as the number of retrieved oocytes, fertilized oocytes and good quality embryos, and the oocyte maturation rate were similar among the three groups. Implantation, biochemical pregnancy and clinical miscarriage rates did not differ significantly were not significantly different among three groups. The clinical pregnancy rate was significantly lower in the high hsCRP group compared with that in the low hsCRP group (50.0% versus 63.4%; P<0.0167), which contributed to a significant decrease in birth rate (39.8% versus 53.8%; P<0.0167). High serum hsCRP levels was found to be a factor affecting live birth rate.

CONCLUSIONS

Among women of normal weight undergoing their first IVF treatment, it was found that low-grade inflammation was associated with reduced clinical pregnancy and live birth rates following fresh ET cycles.

High-sensitivity C-reactive protein levels and pregnancy outcomes in women with unexplained infertility after ovarian stimulation with intrauterine insemination in a multicenter trial

Objective

To determine if chronic inflammation, assessed by basal high-sensitivity C-reactive protein (hs-CRP) levels, is associated with pregnancy outcomes in women with unexplained infertility undergoing ovarian stimulation with intrauterine insemination.

Design

Prospective cohort analysis of the Reproductive Medicine Network's Assessment of Multiple Intrauterine Gestations from Ovarian Stimulation (AMIGOS) randomized controlled trial.

Setting

Multicenter university-based randomized controlled trial.

Patients

A total of 781 couples with unexplained infertility.

Interventions

Secondary analysis.

Main Outcome Measures

Adjusted risk ratios of live birth, clinical pregnancy, and pregnancy loss rates by hs-CRP levels.

Results

Associations between hs-CRP levels and clinical pregnancy rates were not observed after adjustment for baseline body mass index. There were fewer live births among women with higher hs-CRP levels, although confidence intervals crossed 1.0. The risk of pregnancy loss was greater in women with increased hs-CRP levels (1-3 mg/L: risk ratio [RR], 1.67; 95% confidence interval [CI], 1.00-2.79; >3-10 mg/L: RR, 1.84; 95% CI, 1.06-3.20; and >10 mg/L: RR, 2.14; 95% CI, 1.05-4.36 compared to women with hs-CRP <1 mg/L).

Conclusions

This investigation suggests that chronic inflammation may increase the risk of pregnancy loss but not impact the clinical pregnancy rate in women with unexplained infertility undergoing ovarian stimulation with intrauterine insemination. Associations between inflammation and pregnancy outcomes in women with infertility merit further investigation.

Clinical Trial Registration Number

clinicaltrials.gov NCT01044862.

Association between dietary inflammatory index and inflammatory biomarkers with outcomes of in vitro fertilization treatment

AIM

To investigate associations between dietary inflammatory index (DII), high-sensitivity C-reactive protein (hs-CRP) and interleukin-6 (IL-6) with outcomes of in vitro fertilization treatment.

METHODS

This observational prospective study included 144 women undergoing fresh in vitro fertilization cycles who attended infertility center of Al-Zahra Hospital in Tabriz, Iran. DII was computed based on dietary intake assessed using a 3-day 24-h food record diary. Outcomes of in vitro fertilization in this study were considered fertilization rate, embryo quality and positive pregnancy test. Inflammatory biomarkers were measured on the day of embryo transfer.

RESULTS

There was no statistically significant difference between pregnant and nonpregnant women in terms of infertility and demographic characteristics, quantity of retrieved and fertilized oocytes, fertilization rate and number of transferred embryos (P > 0.05). The mean quality scores of resultant embryos (P < 0.001) and transferred embryos (P = 0.019) were significantly lower in the group of pregnant women, indicating the better quality among them. Median (percentile 25-75th) DII in the pregnant women was 1.8 (0.5-2.7) and in the nonpregnant women was 1.6 (0.3-2.6). DII was significantly related to hs-CRP (r = 0.198, P = 0.017) but not to IL-6. There were no significant relationships between DII, serum hs-CRP, IL-6 and in vitro fertilization outcomes (P > 0.05).

CONCLUSION

These findings demonstrate that serum hs-CRP and IL-6 concentration and DII are not predictive markers of in vitro fertilization cycle outcomes in women undergoing in vitro fertilization.

Maternal C-reactive protein and in vitro fertilization (IVF) cycles

Embryo implantation is accompanied by a potent inflammatory response, and a gradient of cytokines and chemokines produced by endometrial cells supports the embryo-endometrial interaction. C-reactive protein (CRP) serves as an early marker of inflammation and recent studies have illustrated that controlled ovarian hyperstimulation (COH) could increase its levels. Interestingly, a high chance of pregnancy has been reported in women who had an elevated CRP level on the day of embryo transfer. It seems an elevated systemic inflammation in the in vitro fertilization (IVF) cycle can increase the implantation and pregnancy rates. However, the results regarding the association of CRP with ART outcomes are controversial. Therefore, in this review, we aimed to describe how CRP levels change during a cycle of IVF treatment and which factors can potentially affect this pattern of change. Furthermore, the association of CRP with ART outcomes has been discussed.

Impact of contraception and IVF hormones on metabolic, endocrine, and inflammatory status

Assisted reproductive technologies (ART) represent commonly utilized management strategies for infertility with multifactorial causes (including genetically predisposed diseases). Amongst ART, in vitro fertilization (IVF) is the most popular. IVF treatment may predispose the mother to increased risks and complications during pregnancy, and there may be adverse fetal outcomes. Hormonal therapies, including oral contraceptives, may impair glucose and lipid metabolism, and promote insulin resistance and inflammation. IVF treatment involves administration of reproductive hormones, similar in composition but in much higher doses than those used for oral contraception. The provision of IVF reproductive hormones to mice associates with glucose intolerance. In addition, the physiological and hormonal changes of pregnancy can trigger an inflammatory response, and metabolic and endocrine changes. There is controversy regarding the potential effects of IVF hormonal therapies in the promotion of diabetogenic and inflammatory states, additional to those that occur during pregnancy, and which may therefore predispose women with IVF-conceived pregnancies to adverse obstetric outcomes compared with women with spontaneously conceived pregnancies. This review summarizes the limited published evidence regarding the effect of IVF-based fertility therapies on glucose homeostasis, insulin resistance, cardio-metabolic profile, and markers of inflammation.

C-reactive protein and ART outcomes: a systematic review

BACKGROUND

A dynamic balance between pro- and anti-inflammatory factors contributes to regulating human female reproduction. Chronic low-grade inflammation has been detected in several female reproductive conditions, from anovulation to embryo implantation failure. C-reactive protein (CRP) is a reliable marker of inflammation that is extensively used in clinical practice. Recent studies quantified CRP in the serum of infertile women undergoing ART and suggested its potential for the prediction of ART reproductive outcomes.

OBJECTIVE AND RATIONALE

The first objective of this systematic review of the available literature was to evaluate the association between pre-implantation circulating CRP concentration and pregnancy rates in women undergoing ART. The second objective was to describe serum CRP concentration changes after early embryo implantation. The changes in circulating CRP throughout the ART cycle, clinical implications of CRP quantification for the management of women undergoing ART, and future therapeutic options will also be discussed.

SEARCH METHODS

The MEDLINE database was systematically searched from inception to March 2019 using the following key words: (C-reactive protein) AND (assisted reproductive techniques OR ovulation induction OR insemination OR in vitro fertilization). Only articles in English were considered. Studies were selected based on title and abstract. The full text of potentially relevant articles was retrieved and assessed for inclusion by two reviewers (S.B. and S.H.). The protocol was registered in the International prospective register of systematic reviews (PROSPERO; registration number: CRD148687).

OUTCOMES

In total, 10 studies were included in this systematic review. Most of these studies reported lower circulating CRP values before the window of implantation and higher circulating CRP values during the peri-implantation period in women with successful ART outcome (biochemical or clinical pregnancy) compared to women without a successful outcome. Several lifestyle factors and/or drugs that reduce the concentration of circulating CRP significantly improve ART outcomes. Subgroup analyses according to female BMI and baseline circulating CRP concentration are highly recommended in future analyses.

WIDER IMPLICATIONS

These findings highlight a possible detrimental impact of preconception high circulating CRP concentration on ART outcomes. However, the biochemical or clinical pregnancy rate endpoints used in the studies examined here are insufficient (there were no data on live birth outcome), and the impact of major variables that can influence CRP and/or ART, for example maternal age, BMI, number of transferred embryos, and use of anti-inflammatory drugs, were not considered in the analyses. CRP quantification may be a potential marker of ART outcome, but its predictive value still needs to be investigated in large prospective studies. In future, the quantification of circulating CRP before starting ART could help to identify patients with a poor ART prognosis, leading to ART cycle cancellation or to preconception treatment to minimize the medical risks and costs.

The effect of serum and follicular fluid secreted frizzle-related protein-5 on in vitro fertilization outcomes in patients with polycystic ovary syndrome

In this study, we aimed to investigate serum and follicular fluid (FF) secreted frizzle-related protein-5 (Sfrp-5) levels in nonobese, nonhyperandrogenic patients with polycystic ovary syndrome (PCOS) undergoing in vitro fertilization (IVF), in addition to IVF outcomes. In total, 160 patients undergoing IVF treatment were included in the study: 80 patients diagnosed with PCOS according to the Rotterdam criteria (group I, study) and 80 patients with the etiology of male factor infertility (group II, control). There were statistically significant between-group differences in serum estradiol (E₂) levels on the day of hCG administration (2377.00 ± 733.23 vs. 1931.3 ± 1,010.69), the total gonadotropin dose required (2000.63 ± 1,051.87 vs. 1.134.69 ± 286.45), and the total number of retrieved oocytes (8.60 ± 2.06 vs. 11.05 ± 4.39) (p < 0.05). There was also a statistically significant between-group difference in serum and FF Sfrp-5 levels on the day of oocyte retrieval (11.40 ± 2.88 vs. 8.87 ± 1.85, p < 0.001; 11.06 ± 2.30 vs. 9.71 ± 2.15, p = 0.008; respectively). However, there were no between-group differences in fertilization rates, clinical pregnancy rates, and live birth rates (p > 0.05). A correlation analysis showed that serum and FF Sfrp-5 levels were associated with insulin and inflammatory markers (p < 0.05). In a selected population of nonobese, nonhyperandrogenic PCOS patients, there was a significant difference in Sfrp-5 levels of the PCOS group versus those of the control group. Further studies are needed to determine the effects of Sfrp-5 in women with PCOS.

C-Reactive protein in relation to fecundability and anovulation among eumenorrheic women

OBJECTIVE

To assess systemic inflammation in relation to fecundability and anovulation.

DESIGN

Prospective cohort study among participants in the Effects of Aspirin in Gestation and Reproduction trial who were assigned to the placebo.

SETTING

Academic medical centers.

PATIENT(S)

Healthy eumenorrheic women (n = 572), 18-40 years of age, with one or two pregnancy losses, attempting spontaneous pregnancy.

INTERVENTION(S)

Baseline serum high-sensitivity C-reactive protein (hsCRP) values <10 mg/L were categorized into tertiles.

MAIN OUTCOME MEASURE(S)

Discrete Cox proportional hazards models estimated the fecundability odds ratio (FOR) and 95% confidence interval (CI) and adjusted for potential confounders. Log-binomial regression estimated the risk ratio (RR) and 95% CI of anovulation. The algorithm to define anovulation used data on urinary concentrations of hCG, pregnanediol-3-glucuronide, and LH as well as fertility monitor readings.

RESULT(S)

Higher hsCRP was associated with reduced fecundability but not with an increased risk of anovulation.

CONCLUSION(S)

Among healthy women attempting pregnancy after one or two pregnancy losses, we found preliminary evidence that systemic inflammation is associated with reduced fecundability, but not independently from adiposity. Sporadic anovulation did not appear to drive this association.

CLINICAL TRIAL REGISTRATION NUMBER

ClinicalTrials.gov: NCT00467363.

Preconception Low-Dose Aspirin Restores Diminished Pregnancy and Live Birth Rates in Women With Low-Grade Inflammation: A Secondary Analysis of a Randomized Trial

Context

Inflammation is linked to causes of infertility. Low-dose aspirin (LDA) may improve reproductive success in women with chronic, low-grade inflammation.

Objective

To investigate the effect of preconception-initiated LDA on pregnancy rate, pregnancy loss, live birth rate, and inflammation during pregnancy.

Design

Stratified secondary analysis of a multicenter, block-randomized, double-blind, placebo-controlled trial.

Setting

Four US academic medical centers, 2007 to 2012.

Participants

Healthy women aged 18 to 40 years (N = 1228) with one to two prior pregnancy losses actively attempting to conceive.

Intervention

Preconception-initiated, daily LDA (81 mg) or matching placebo taken up to six menstrual cycles attempting pregnancy and through 36 weeks' gestation in women who conceived.

Main Outcome Measures

Confirmed pregnancy, live birth, and pregnancy loss were compared between LDA and placebo, stratified by tertile of preconception, preintervention serum high-sensitivity C-reactive protein (hsCRP) (low, <0.70 mg/L; middle, 0.70 to <1.95 mg/L; high, ≥1.95 mg/L).

Results

Live birth occurred in 55% of women overall. The lowest pregnancy and live birth rates occurred among the highest hsCRP tertile receiving placebo (44% live birth). LDA increased live birth among high-hsCRP women to 59% (relative risk, 1.35; 95% confidence interval, 1.08 to 1.67), similar to rates in the lower and mid-CRP tertiles. LDA did not affect clinical pregnancy or live birth in the low (live birth: 59% LDA, 54% placebo) or midlevel hsCRP tertiles (live birth: 59% LDA, 59% placebo).

Conclusions

In women attempting conception with elevated hsCRP and prior pregnancy loss, LDA may increase clinical pregnancy and live birth rates compared with women without inflammation and reduce hsCRP elevation during pregnancy.

The effect of serum and follicular fluid amyloid-associated protein levels on in vitro fertilization outcome in patients with polycystic ovary syndrome

PURPOSE

In this study, we aimed to investigate serum and follicular fluid amyloid A protein levels in non-obese non-hyperandrogenic patients with polycystic ovary syndrome (PCOS) undergoing in vitro fertilization (IVF) and IVF outcome.

METHODS

A total of 81 patients undergoing IVF treatment, 41 patients diagnosed as PCOS according to the Rotterdam criteria (group I) and 40 patients with the etiology of male factor infertility (group II), were included in the study. On the day of oocyte pickup, serum and follicular fluid samples were collected from all patients.

RESULTS

Serum E2 level on the day of hCG (2849.93 ± 541.54 vs. 2494.28 ± 712.98) and total number of retrieved oocytes (13.73 ± 3.57 vs. 10.53 ± 4.07) were significantly higher in group I when compared to group II (p < 0.05). However, number of mature oocytes, fertilization rate, and clinical pregnancy rate did not differ (p > 0.05). No significant difference was found between two groups regarding the serum and follicular fluid amyloid A protein levels on the day of oocyte retrieval (p > 0.05).

Evaluation of human chaperonin 10 and high-sensitivity C-reactive protein levels of infertile women who underwent ovulation induction and intra-uterine insemination

INTRODUCTION

The implantation of embryo is one of the crucial steps of a successful pregnancy. The foetus should be protected from maternal immune system, for the appropriate implantation and modification in maternal immunity is crucial. We investigated high-sensitivity C-reactive protein (hs CRP), which is an indicator of low-grade inflammation and Cp10 that has immunosuppressant and growth-promoting capabilities at embryo levels in ovulation induction and intra-uterine insemination (IUI)applied in infertile women. The ovulation induction was maintained by clomiphene citrate or gonadotropins for 42 infertile patients. After successful ovulation induction, IUI was carried out. The blood samples were taken 2 and 8 days after IUI to evaluate Cp10 and hs CRP levels. The pregnant and non-pregnant groups' results were analyzed. The Cp10 levels 8 days after IUI were higher in pregnant group, whereas there was no difference for the 2 days after levels between pregnant and non-pregnant group. The hs CRP levels were similar for both 2nd and 8th days when we compared pregnant and non-pregnant groups. The Cp10 levels increased from day 2 to day 8 in pregnant group. In contrast, the Cp10 levels decreased in non-pregnant group. The change in hs CRP levels from day 2 to day 8 was not significant in pregnant and non-pregnant groups. The Cp10 levels were higher in early phases of fertilisation and elevated through the preceding days of conception in pregnant patients, while it decreased in non-pregnant patients with failed cycles.

The relationship between the change in serum high sensitivity C-reactive protein levels and IVF success

The aim of this study was to assess whether the extent of the change in high sensitivity C-reactive protein (hs-CRP) levels is related with in vitro fertilization (IVF) success. A total of 69 IVF cycles using long luteal GnRH agonist protocol at the IVF unit, were prospectively studied. The serum levels of hs-CRP were measured on the day of initiation of gonadotrophin stimulation and 7 days after embryo transfer. CRP ratio was defined as the levels of CRP on day 7 of transfer/day of initiation of ovarian stimulation. Clinical pregnancy rates were examined. The mean concentrations of hs-CRP were not significantly different on the first day of ovarian stimulation and on day 7 after embryo transfer among pregnant and non-pregnant women. There was a significant rise in hs-CRP levels at 7th day after embryo transfer as compared with the first day of gonadotrophin treatment in both groups (10.58 ± 11.35 versus 3.61 ± 2.86 mg/L for pregnant women and 9.14 ± 11.36 versus 3.24 ± 2.68 mg/L for non-pregnant women, p = 0.001). In addition, the mean CRP ratio was not different between the pregnant and non-pregnant groups. Our data show that serum hs-CRP levels increase during IVF treatment, but the extent of the rise in CRP levels is not a predictive marker of IVF success.

Reproductive ecology and the endometrium: physiology, variation, and new directions

Endometrial function is often overlooked in the study of fertility in reproductive ecology, but it is crucial to implantation and the support of a successful pregnancy. Human female reproductive physiology can handle substantial energy demands that include the production of fecund cycles, ovulation, fertilization, placentation, a 9-month gestation, and often several years of lactation. The particular morphology of the human endometrium as well as our relative copiousness of menstruation and large neonatal size suggests that endometrial function has more resources allocated to it than many other primates. The human endometrium has a particularly invasive kind of hemochorial placentation and trophoblast that maximizes surface area and maternal-fetal contact, yet these processes are actually less efficient than the placentation of some of our primate relatives. The human endometrium and its associated processes appear to prioritize maximizing the transmission of oxygen and glucose to the fetus over efficiency and protection of maternal resources. Ovarian function controls many aspects of endometrial function and thus variation in the endometrium is often a reflection of ecological factors that impact the ovaries. However, preliminary evidence and literature from populations of different reproductive states, ages and pathologies also suggests that ecological stress plays a role in endometrial variation, different from or even independent of ovarian function. Immune stress and psychosocial stress appear to play some role in the endometrium's ability to carry a fetus through the mechanism of inflammation. Thus, within reproductive ecology we should move towards a model of women's fecundity and fertility that includes many components of ecological stress and their effects not only on the ovaries, but on processes related to endometrial function. Greater attention on the endometrium may aid in unraveling several issues in hominoid and specifically human evolutionary biology: a low implantation rate, high rates of early pregnancy loss, prenatal investment in singletons but postnatal support of several dependent offspring at once, and higher rate of reproductive and pregnancy-related pathology compared to other primates, ranging from endometriosis to preeclampsia. The study of the endometrium may also complicate some of these issues, as it raises the question of why humans have a maximally invasive placentation method and yet slow fetal growth rates. In this review, I will describe endometrial physiology, methods of measurement, variation, and some of the ecological variables that likely produce variation and pregnancy losses to demonstrate the necessity of further study. I propose several basic avenues of study that leave room for testable hypotheses in the field of reproductive ecology. And finally, I describe the potential of this work not just in reproductive ecology, but in the resolution of broader women's health issues.