Socioeconomic status and fecundability in a Danish preconception cohort

Association between fatty acids and female infertility: dual evidence from a cross-sectional study and Mendelian randomization analysis

Background: Infertility poses a considerable threat to female reproductive health on a global scale. Dietary pattern, as a modifiable lifestyle factor, is frequently recommended as an important intervention for infertility-related diseases. Fatty acids play a crucial role in maintaining the health of the female reproductive system. However, the available evidence on the specific relationship between various types of fatty acids and infertility remains insufficient and controversial. Methods: Initially, a cross-sectional study was conducted utilizing the National Health and Nutrition Examination Survey (NHANES) database to collect data from women aged 18-45 years who met the inclusion criteria across the 2013-2020 cycles. Infertility was defined based on information gleaned from reproductive questionnaires. Fatty acid intake was determined by analyzing two 24 hour dietary recall interviews. Weighted logistic regression and weighted restricted cubic spline (RCS) analyses, incorporating covariate adjustments, were employed to preliminarily delineate the association between various types of fatty acids and proportions of fatty acid intake and female infertility risk. Model performance evaluation was carried out through receiver operating characteristic (ROC) curve analysis, complemented by the utilization of a nomogram diagram to gauge the infertility risk attributed to covariates. Genetic instrumental variables pertinent to diverse fatty acid profiles and female infertility were sourced from genome-wide association studies (GWAS). Mendelian randomization (MR), multivariable MR (MVMR) and reverse MR analyses were subsequently used to ascertain causality and reverse causality between distinct fatty acids and infertility, concurrently assessing for heterogeneity and horizontal pleiotropy. Results: In our NHANES analysis, a total of 3159 women were enrolled in the study, with a self-reported infertility prevalence of 11.49%. Infertile women exhibited significantly elevated intake of total omega-6 and omega-6/total fatty acids (TFA) compared to the controls. Weighted logistic regression models confirmed positive correlations between total omega-6 (continuous) and omega-6/TFA (categorical) and infertility risk, while omega-3 (continuous) intake demonstrated a negative correlation. Model 2, post rigorous multivariate covariate adjustment, showed improved predictive performance according to ROC curve analysis. Subgroup analysis suggested that the positive correlation between omega-6/TFA (continuous) and female infertility risk was not affected by stratification. Total omega-6 (continuous) emerged as a risk factor for infertile women aged 18-34 years. However, total saturated fatty acids (TSFAs, continuous), total omega-3 (continuous) and total polyunsaturated fatty acids (PUFAs, categorical) were protective factors only in the infertile women with a BMI ≥ 25 kg m-2. The positive associations between total omega-6 (Q4) and omega-6/TFA (continuous and Q3-Q4) and infertility risk were consistent across all BMI subgroups. MR analysis employing inverse variance weighted (IVW) as the primary method and Bonferroni correction revealed that genetically predicted TSFAs, monounsaturated fatty acids (MUFAs), omega-6 and MUFA/TFA were positively associated with female infertility risk, whereas PUFA/TFA showed a negative association. Importantly, the positive associations between MUFAs and omega-6 and infertility risk remained robust even after adjusting for potential confounders using MVMR analyses. Reverse MR analysis did not provide any evidence for reverse causality. The MR-Egger regression intercept and Cochran's Q test did not detect any heterogeneity or horizontal pleiotropy. Conclusions: This study presents compelling evidence to substantiate the link between diverse fatty acids, particularly omega-6 PUFAs, and the risk of female infertility. However, to fully comprehend the potential mechanisms and impact of distinct fatty acids and their compositional ratios on female infertility, extensive future research spanning fundamental and large-scale clinical inquiries is imperative.

The association between the use of fertility indicators and fecundability in a Danish preconception cohort

BACKGROUND

The use of fertility indicators to predict ovulation has largely been studied for contraceptive purposes, while less so as fertility-promoting tools.

OBJECTIVE

To investigate the association between fertility indicators and fecundability in Danish women trying to conceive.

METHODS

Web-based preconception cohort study. We analysed data from 11,328 females aged 18-49 years trying to conceive without fertility treatment for ≤6 menstrual cycles, from the Danish SnartGravid.dk and SnartForældre.dk cohorts (2007-2023). Participants reported the use of fertility indicators (counting days since the last menstrual period, cervical fluid monitoring, urinary ovulation testing, feeling ovulation, using a smartphone fertility app and measuring basal body temperature [BBT]). Time to pregnancy was measured in menstrual cycles ascertained by self-reported pregnancy status. We estimated fecundability ratios (FR) and 95% confidence intervals (CI) using proportional probabilities regression models adjusted for age, socio-economic position, health indicators, reproductive history and gynaecological factors.

RESULTS

Fertility indicators were used by 63.3% of participants at study entry. Counting days was the most common (46.9%), while measuring BBT was the least (3.0%). Other indicators ranged from 17.0% to 23.6%, with 69.7% using more than one indicator. Compared with non-use, use of any fertility indicator was associated with greater fecundability (adjusted FR 1.14, 95% CI 1.08, 1.19). Cervical fluid monitoring showed the strongest association (aFR 1.46, 95% CI 1.03, 2.07), followed by urinary ovulation testing (aFR 1.35, 95% CI 1.16, 1.58) and counting days (aFR 1.18, 95% CI 1.09, 1.29). Feeling ovulation and fertility apps were modestly associated with fecundability, while measuring BBT was not associated. Sensitivity analysis restricting to ≤2 cycles of attempt time and two cycles of follow-up showed an aFR for any indicator use of 1.21 (95% CI 1.13, 1.31).

CONCLUSION

In this Danish preconception cohort, use of fertility indicators was associated with a higher fecundability, varying by type of indicator.

Time to pregnancy and life expectancy: a cohort study of 18 796 pregnant couples

STUDY QUESTION

Is fecundity, measured as time to pregnancy (TTP), associated with mortality in parents?

SUMMARY ANSWER

Prolonged TTP is associated with increased mortality in both mothers and fathers in a dose-response manner.

WHAT IS KNOWN ALREADY

Several studies have linked both male and female fecundity to mortality. In women, infertility has been linked to several diseases, but studies suggest that the underlying conditions, rather than infertility, increase mortality.

STUDY DESIGN, SIZE, DURATION

A prospective cohort study was carried out on 18 796 pregnant couples, in which the pregnant women attended prophylactic antenatal care between 1973 and 1987 at a primary and tertiary care unit. The couples were followed in Danish mortality registers from their child's birth date until death or until 2018. The follow-up period was up to 47 years, and there was complete follow-up until death, emigration or end of study.

PARTICIPANTS/MATERIALS, SETTING, METHODS

At the first antenatal visit, the pregnant women were asked to report the time to the current pregnancy. Inclusion was restricted to the first pregnancy, and TTP was categorised into <12 months, ≥12 months, not planned, and not available. In sub-analyses, TTP ≥12 was further categorized into 12-35, 36-60, and >60 months. Information for parents was linked to several Danish nationwide health registries. Survival analysis was used to estimate the hazard ratios (HRs) with a 95% CI for survival and adjusted for age at the first attempt to become pregnant, year of birth, socioeconomic status, mother's smoking during pregnancy, and mother's BMI.

MAIN RESULTS AND THE ROLE OF CHANCE

Mothers and fathers with TTP >60 months survived, respectively, 3.5 (95% CI: 2.6-4.3) and 2.7 (95% CI: 1.8-3.7) years shorter than parents with a TTP <12 months. The mortality was higher for fathers (HR: 1.21, 95% CI: 1.09-1.34) and mothers (HR: 1.29, 95% CI: 1.12-1.49) with TTP ≥12 months compared to parents with TTP <12 months. The risk of all-cause mortality during the study period increased in a dose-response manner with the highest adjusted HR of 1.98 (95% CI: 1.62-2.41) for fathers and 2.03 (95% CI: 1.56-2.63) for mothers with TTP >60 months. Prolonged TTP was associated with several different causes of death in both fathers and mothers, indicating that the underlying causes of the relation between fecundity and survival may be multi-factorial.

LIMITATIONS, REASONS FOR CAUTION

A limitation is that fecundity is measured using a pregnancy-based approach. Thus, the cohort is conditioned on fertility success and excludes sterile couples, unsuccessful attempts and spontaneous abortions. The question used to measure TTP when the pregnant woman was interviewed at her first attended prophylactic antenatal care: 'From the time you wanted a pregnancy until it occurred, how much time passed?' could potentially have led to serious misclassification if the woman did not answer on time starting unprotected intercourse but on the start of wishing to have a child.

WIDER IMPLICATIONS OF THE FINDINGS

We found that TTP is a strong marker of survival, contributing to the still-emerging evidence that fecundity in men and women reflects their health and survival potential.

STUDY FUNDING/COMPETING INTEREST(S)

The authors acknowledge an unrestricted grant from Ferring. The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article, or the decision to submit it for publication. M.L.E. is an advisor to Ro, VSeat, Doveras, and Next.

TRIAL REGISTRATION NUMBER

N/A.