Risk of Adverse Obstetric Outcomes and the Abnormal Growth of Offspring in Women with a History of Thyroid Cancer

Risk of adverse obstetric outcomes in patients with a history of endometrial cancer: A nationwide population-based cohort study

OBJECTIVE

To evaluate adverse obstetric outcomes in women with a history of endometrial cancer (EC).

DESIGN

Population-based cohort study.

SETTING

The Korean National Health Insurance (KNHI) claims database.

POPULATION

Women who gave birth between 2009 and 2016, with a history of EC prior to pregnancy.

METHODS

The KNHI database was used to compare obstetric outcomes of women with and without a history of EC, using the ICD-10 codes. Multivariable logistic regression models were used to determine the associations between a history of EC and adverse obstetric outcomes.

MAIN OUTCOMES MEASURES

Adverse obstetric outcomes.

RESULTS

Overall, 248 and 3 335 359 women with and without a history of EC, respectively, gave birth. When adjusted for age, primiparity and comorbidities, an increased risk of multiple gestations (odds ratio [OR] 4.925, 95% confidence interval [CI] 3.394-7.147), caesarean delivery (OR 2.005, 95% CI 1.535-2.62) and preterm birth (OR 1.941, 95% CI 1.107-3.404) was observed among women with a history of EC. We were unable to demonstrate significant differences in the risk of pre-eclampsia, gestational diabetes, vacuum delivery, placenta praevia, placenta accreta spectrum, placental abruption and postpartum haemorrhage between the groups. In the sensitivity analyses excluding multiple gestations, an increased risk of preterm birth was not observed among women with a history of EC (OR 1.276, 95% CI 0.565-2.881).

CONCLUSIONS

There is no convincing evidence of an increased risk of adverse obstetric outcomes among women with a history of EC. Our findings would be useful in counselling of patients with EC who are undergoing fertility-sparing treatment.

Adverse birth outcomes among offspring born to women diagnosed with cancer: a population-based cohort study

BACKGROUND

With increasing cancer incidence and survival rates, the prevalence of maternal cancer and its effect on adverse birth outcomes are important for prenatal care and oncology management. However, the effects of different types of cancer at different gestational stages have not been widely reported.

OBJECTIVE

This study aimed to describe the epidemiologic characteristics of pregnancy-associated cancers (during and 1 year after pregnancy) and evaluate the association between adverse birth outcomes and maternal cancers.

METHODS

Of 983,162 cases, a history of maternal cancer, including pregestational cancer, pregnancy-associated cancer, and subsequent cancer, was identified in 16,475 cases using a health information network. The incidence and 95% confidence interval of pregnancy-associated cancer were calculated with the Poisson distribution. The adjusted risk ratio with 95% confidence interval of the association between adverse birth outcomes and maternal cancer were estimated using the multilevel log-binomial model.

RESULTS

A total of 38,295 offspring were born to mothers with a cancer history. Of these, 2583 (6.75%) were exposed to pregnancy-associated cancer, 30,706 (80.18%) had a subsequent cancer diagnosis, and 5006 (13.07%) were exposed to pregestational cancer. The incidence of pregnancy-associated cancer was 2.63 per 1000 pregnancies (95% confidence interval, 2.53‰-2.73‰), with cancer of the thyroid (1.15‰), breast (0.25‰), and female reproductive organs (0.23‰) being the most common cancer types. The increased risks of preterm birth and low birthweight were significantly associated with cancer diagnosed during the second and third trimester of pregnancy, whereas increased risks of birth defects (adjusted risk ratio, 1.48; 95% confidence interval, 1.08-2.04) were associated with cancer diagnosed in the first trimester. Increased risks of preterm birth (adjusted risk ratio, 1.16; 95% confidence interval, 1.02-1.32), low birthweight (adjusted risk ratio, 1.24; 95% confidence interval, 1.07-1.44), and birth defects (adjusted risk ratio, 1.22; 95% confidence interval, 1.10-1.35) were observed in thyroid cancer survivors.

CONCLUSION

Careful monitoring of fetal growth should be implemented for women diagnosed with cancer in the second and third trimester to ensure timely delivery and balance the benefits of neonatal health and cancer treatment. The higher incidence of thyroid cancer and increased risk of adverse birth outcomes among thyroid cancer survivors suggested that the regular thyroid function monitoring and regulation of thyroid hormone levels are important in maintaining pregnancy and promoting fetal development among thyroid cancer survivors before and during pregnancy.

Pregnancy outcomes and neonatal thyroid function in women with thyroid cancer: a retrospective study

BACKGROUND

Evidence regarding adverse pregnancy outcomes in patients with thyroid cancer has been conflicting, and the effect of thyroid dysfunction caused by thyroid hormone suppression therapy in terms of neonatal thyroid stimulating hormone (TSH) is unclear. This study aimed to investigate whether thyroid cancer was associated with adverse pregnancy outcomes and had an adverse effect on neonatal thyroid function.

METHODS

This was a retrospective study of 212 singleton pregnancies with thyroid cancer and 35,641 controls without thyroid cancer. Data on maternal pregnancy outcomes and neonatal outcomes were analyzed.

RESULTS

The median TSH level in the thyroid cancer group was significantly lower than that in the control group (0.87 µIU/mL vs. 1.17 µIU/mL; P < 0.001), while the FT4 level was higher than that in the control group (17.16 pmol/L vs. 16.33 pmol/L; P < 0.001). The percentage of thyroid peroxidase antibodies (TPOAb) positive in the thyroid cancer group was significantly higher than that in the control group (25.0% vs. 11.8%; P < 0.001). Pregnancies with thyroid cancer had a higher risk of late miscarriage (OR 7.166, 95% CI: 1.521, 33.775, P = 0.013), after adjusting maternal TPOAb positive, there was no statistical significance (OR 3.480, 95% CI: 0.423, 28.614, P = 0.246). Pregnancies with thyroid cancer had higher gestational weight gain (GWG) (14.0 kg vs. 13.0 kg, P < 0.001). Although there was no significant difference in the prevalence of gestational diabetes mellitus (GDM) (20.8% vs. 17.4%, P = 0.194), the oral glucose tolerance test (OGTT) showed that fasting plasma glucose and 2-hour value in the thyroid cancer group were higher than those in the control group (P = 0.020 and 0.004, respectively). There was no statistically significant difference in TSH between the thyroid cancer group and the control group, regardless of full-term newborns or preterm newborns.

CONCLUSIONS

Thyroid cancer might not have substantial adverse effects on pregnancy outcomes except for excessive GWG. No adverse effect on neonatal TSH was found, but the effect on long-term thyroid function and neuropsychological function in offspring need further study.

TRIAL REGISTRATION

Beijing Birth Cohort Study (ChiCTR220058395).

Impact of endocrine disorders on stillbirth: a prospective cohort study

OBJECTIVE

Among risk factors for SB, maternal endocrine diseases (ED), such as thyroids dysfunction and gestational diabetes mellitus (GDM), are the most frequent. This study aimed to investigate the rate of ED in a population of SB cases collected prospectively, and the relationship between these and causes of death.

METHODS

This is an area-based, prospective cohort study conducted in Emilia-Romagna, Italy between January 2014 and December 2020. Data included all cases of SB (>22 weeks).

RESULTS

From 2014 to 2020, 766 SB occurred out of a total of 232.506 births (SB rate:0.3/1000). The ED were present in 197/766 cases of SB (25.7%), respectively, 104 thyroid disease (52.8%), 74 GDM (37.5%), and 19 cases of concomitant GDM and thyroid disease (9.6%). Women who had SB associated with ED presented significantly higher mean maternal age (p < 0.001), BMI (p < 0.001), obesity (p < 0.001) and lower smoking habit (p = 0.02) respect with control group. Neonatal and placental weight of stillborn women with ED was significantly higher (p < 0.001) in respect to stillborn of the control group. Women with ED as associated condition (ReCODE classification), present significantly higher cases of SB caused by placenta pathologies (p = 0.009) namely abruptio placentae (p = 0.001) respect than control group.

CONCLUSIONS

ED was more frequent in older and obese women experiencing SB. The main cause of death was abruptio placentae. This information can be helpful when counseling mothers with ED and planning antenatal management to prevent SB.

Diagnostic and Treatment Considerations for Thyroid Cancer in Women of Reproductive Age and the Perinatal Period

Thyroid cancer is one of the most common cancers diagnosed in women of reproductive age and during pregnancy. This leads to important questions about thyroid cancer prognosis and treatment, but also fertility and risk for adverse obstetric and/or fetal and neonatal outcomes. The benefits of thyroid cancer treatment should be weighed against its harms, as various options may adversely impact maternal and fetal health. In the current review, the authors focus on perinatal-specific clinical considerations related to the care of patients with thyroid cancer.

Assessment and treatment of thyroid disorders in pregnancy and the postpartum period

Thyroid disorders are prevalent in pregnant women. Furthermore, thyroid hormone has a critical role in fetal development and thyroid dysfunction can adversely affect obstetric outcomes. Thus, the appropriate management of hyperthyroidism, most commonly caused by Graves disease, and hypothyroidism, which in iodine sufficient regions is most commonly caused by Hashimoto thyroiditis, in pregnancy is important for the health of both pregnant women and their offspring. Gestational transient thyrotoxicosis can also occur during pregnancy and should be differentiated from Graves disease. Effects of thyroid autoimmunity and subclinical hypothyroidism in pregnancy remain controversial. Iodine deficiency is the leading cause of hypothyroidism worldwide. Despite global efforts to eradicate iodine deficiency disorders, pregnant women remain at risk of iodine deficiency due to increased iodine requirements during gestation. The incidence of thyroid cancer is increasing worldwide, including in young adults. As such, the diagnosis of thyroid nodules or thyroid cancer during pregnancy is becoming more frequent. The evaluation and management of thyroid nodules and thyroid cancer in pregnancy pose a particular challenge. Postpartum thyroiditis can occur up to 1 year after delivery and must be differentiated from other forms of thyroid dysfunction, as treatment differs. This Review provides current evidence and recommendations for the evaluation and management of thyroid disorders in pregnancy and in the postpartum period.

Pregnancy Outcomes in Thyroid Cancer Survivors: A Propensity Score-Matched Cohort Study

BACKGROUND

Some female thyroid cancer survivors wish to become pregnant following their cancer treatment. Current studies have shown inconsistent results on pregnancy outcomes in these survivors; however, detailed information on the pathological type, treatment, and gestational thyroid function of these patients are not yet well documented, making the refined assessment of the influence of a history of thyroid cancer and related treatments on pregnancy outcomes challenging.

OBJECTIVE

To investigate the risk of adverse pregnancy outcomes in thyroid cancer survivors.

METHODS

This was a retrospective cohort study. We included all women aged between 19 and 45 years old who delivered between January 2019 and June 2020 in West China Second University Hospital of Sichuan University. Women with tumors other than thyroid cancer or other thyroid diseases were excluded. The included women were divided into survivors of thyroid cancer (survivors) and women without any history of thyroid disease (controls). Propensity score matching and logistic regression were used to control confounding variables.

RESULTS

All 18,332 women who met the inclusion criteria were included in the study (96 survivors of papillary thyroid cancer and 18,236 controls). After propensity score matching, 96 survivors and 192 controls were included. The survivors had higher levels of free thyroxine (15.47 [13.61-17.67] vs. 14.38 [13.20-15.81] pmol/mL; P<0.001) and higher levels of thyroid peroxidase antibodies (TPOAb) (43.55 [31.43-71.43] vs. 35.95 [28.00-48.03] U/mL; P=0.008) but similar levels of thyroid stimulating hormone (1.46 [0.56-3.15] vs. 1.36 [0.81-1.92] mIU/mL; P=0.142) than the controls. There were no significant differences in adverse pregnancy outcomes between survivors and controls. Fetal macrosomia was lower among survivors (OR: 0.077, 95% CI: 0.009-0.668. P=0.020) than controls. Additionally, survivors had reduced weight gain during pregnancy (13.0 [10.0-15.0] vs. 14.00 [11.00-16.00] kg, P=0.005) and reduced placental weight (563.0 [514.5-620.0] vs. 572.0 [520.0-650.0] g, P=0.019), albeit with small absolute differences. Thyroidectomy or radioiodine therapy did not adversely affect pregnancy outcomes.

CONCLUSION

A history of treated papillary thyroid cancer was not associated with adverse pregnancy outcomes.

Macrosomia and large for gestational age in Asia: One size does not fit all

Macrosomia, usually defined as infant birth weight of ≥4000 g, does not consider gestational age, sex, or country/region-specific differences in mean birth weight and maternal body weight. This issue is particularly relevant for Asia, where 60% of the world's population lives, due to variations in maternal size and birth weights across populations. Large for gestational age (LGA), defined as birth weight > 90th centile, is a more sensitive measure as it considers gestational age and sex, though it is dependent on the choice of growth charts. We aimed to review reporting of macrosomia and LGA in Asia. We reviewed the literature on prevalence and risk of macrosomia and LGA in Asia over the last 29 years. Prevalence of macrosomia ranged from 0.5% (India) to 13.9% (China) while prevalence of LGA ranged from 4.3% (Korea) to 22.1% (China), indicating substantial variation in prevalence within and between Asian countries. High pre-pregnancy body mass index, excessive gestational weight gain, and impaired glucose tolerance conferred risk of macrosomia/LGA. Incidence of macrosomia and LGA varies substantially within and between Asian countries, as do the growth charts and definitions. The latter makes it impossible to make comparisons but suggests differences in intrauterine growth between populations. Reporting LGA, using standardized country/regional growth charts, would better capture the incidence of high birth weight and allow for comparison and identification of contributing factors. Better understanding of local drivers of excessive intrauterine growth could enable development of improved strategies for prevention and management of LGA.

Analysis of in vitro fertilization/intracytoplasmic sperm injection outcomes in infertile women with a history of thyroid cancer: a retrospective study

BACKGROUND

Recent studies have revealed that women with infertility have a higher risk of thyroid cancer (TC) than fertile women. However, studies on whether a history of thyroid cancer affects clinical outcomes in women who conceive using in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) are scarce. We investigate whether a history of thyroid cancer (TC) affects the in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) outcomes and increases the risk of adverse obstetric outcomes in women with infertility.

METHODS

This retrospective study enrolled 384 women with infertility who underwent their first IVF/ICSI treatment at the Peking University Third Hospital between 2010 and 2019. Participants were divided into the TC (64 women with TC history) and control (320 women matched from 85,272 women without thyroid diseases) groups. Controls were individually matched to the TC group according to age, body mass index, concomitant infertility factors, first IVF/ICSI dates, and controlled ovarian stimulation and embryo transfer procedure protocols. IVF/ICSI outcomes, including the numbers of retrieved oocytes and high-grade embryos, clinical pregnancy, miscarriage, preterm delivery, and live birth rates, and adverse obstetric outcome risk were assessed.

RESULTS

The TC group had significantly higher thyroid hormone and lower thyroid-stimulating hormone (TSH) levels than the control group. Despite similar gonadotropin treatment dosage, the TC group had a significantly lower numbers of retrieved oocytes and high-grade embryos than the control group. The occurrence rates of clinical pregnancy, miscarriage, preterm delivery, live births, and adverse obstetric outcomes, including multiple gestation, preterm delivery, gestational diabetes mellitus, gestational hypertension, low birth weight, and large-for-gestational-age infants, were not significantly different between the two groups.

CONCLUSIONS

TC history did not affect the pregnancy outcomes or increase the risk of adverse obstetric outcomes after the first IVF/ICSI, but it may decrease the number of retrieved oocytes and high-grade embryos.

Obstetric outcomes in thyroid cancer survivors: A retrospective cohort study

OBJECTIVE

To investigate whether thyroid cancer survivors would have increased risks of adverse obstetric outcomes.

METHODS

Data from 154 women with a history of thyroid cancer who had a live birth at Peking University Third Hospital from January 2012 to December 2019 were collected. The control group consisted of 308 women without any thyroid disease or malignant tumor. Age and year of delivery were matched between the two groups.

RESULTS

Serum levels of thyroid stimulating hormone and free thyroxine were significantly lower and higher in thyroid cancer patients, respectively (P < 0.001). After adjusting for age, pre-pregnancy body mass index, pre-pregnancy hypertension, pre-pregnancy diabetes mellitus, previous cesarean section, and conception by in vitro fertilization/intracytoplasmic injection, adverse obstetric outcomes including preterm birth (odds ratio [OR] 0.73, 95% confidence interval [CI] 0.32-1.67), low birth weight infant (OR 1.05, 95% CI 0.44-2.50), gestational diabetes mellitus (OR 0.86, 95% CI 0.54-1.39), and pre-eclampsia (OR 1.11, 95% CI 0.34-3.57) showed no differences between thyroid cancer survivors and the control group. However, pregnant women with a history of thyroid cancer had a higher risk of abnormally invasive placentation (OR 10.57, 95% CI 1.22-91.97, P = 0.032).

CONCLUSION

Although the thyroid function status of thyroid cancer patients was different from that of pregnant women without any thyroid disease or malignancy, they would not be at a higher risk of most adverse obstetric outcomes except for abnormally invasive placentation.

A narrative review of pregnancy after malignancies in young women that don't originate in the female genital organs or in the breast

While cancer during pregnancy and its treatment has grown to be a popular topic in recent years, little is known on how to advise patients looking to conceive or conceiving after cancer treatment. The aim of this paper is to review the available literature on the impact of pregnancy on survivors of the most common childhood cancers, brain cancer, haematological malignancies, thyroid cancer, melanomas and sarcomas. Its main objective is to be a source of information for clinicians looking to counsel patients in these delicate moments exploiting all the available literature, albeit scarce. Given the available literature, we conclude that the presence of a multidisciplinary team is of great importance in supporting the patient and her loved ones when facing pregnancy with a previous cancer diagnosis.

Thyroxine Supplementation in Pregnant Women After Thyroidectomy for Thyroid Cancer and Neonatal Birth Weight

Patients are often supplemented with a sufficient dose of thyroxine after thyroidectomy for thyroid cancer. However, the influence of thyroxine supplementation on fetal growth in pregnant women after thyroidectomy for thyroid cancer remains unclear. The aim of this study was to investigate the effect of thyroxine supplementation on neonatal birth weight. This cohort study included 49,896 pregnant women (278 patients with a history of thyroidectomy for thyroid cancer and 39,363 control cases after exclusion). Thyroid parameters were examined in pregnant women and their newborns. The associations between maternal thyroid function and neonatal birth weight and small for gestational age were studied using regression analyses. In the levothyroxine supplementation group, free thyroxine (FT4) levels were significantly higher in both early pregnancy (P < 0.001) and late pregnancy (P < 0.001) groups than in the control group. Furthermore, levels of neonatal thyroid stimulating hormone (P = 0.032) and birth weight (P = 0.043) were significantly lower than those in the control group. We also observed a significant inverse association between maternal FT4 levels in early pregnancy and neonatal birth weight (P=0.028), especially in male newborns (P=0.036). In summary, after thyroidectomy for thyroid cancer, a sufficient dose of thyroxine supplementation in early pregnancy is significantly associated with reduced birth weight and may need to be monitored.

A national database analysis for factors associated with thyroid cancer occurrence

In order to analyze the associations between thyroid cancer and environmental factors, we analyzed the national sample cohort representative of the entire population provided by the Korean National Health Insurance Service database record from 2006 to 2015. The cohort was categorized according to age, body mass index, income, residential areas, frequency of exercise, frequency of alcohol drinking, diet, presence or absence of hyperthyroidism, presence or absence of hypothyroidism, and smoking data. Age ≥ 55 years (HR 0.68, 95% CI 0.53–0.88), lower income (0.57, 0.40–0.80), and current smoking (0.69, 0.55–0.85) were associated with lower thyroid cancer occurrence among men. Body mass index (BMI) ≥ 25 kg/m2 (1.51, 1.26–1.82), higher income (1.44, 1.19–1.76), urban residence (1.24, 1.03–1.49), and presence of hypothyroidism (3.31, 2.38–4.61) or hyperthyroidism (2.46, 1.75–3.46) were associated with higher thyroid cancer occurrence among men. Age ≥ 55 years (0.63, 0.56–0.71), moderate alcohol drinking (0.87, 0.77–0.99), and current smoking (0.56, 0.37–0.85) were associated with lower thyroid cancer occurrence among women. BMI ≥ 25 kg/m2 (1.41, 1.26–1.57), frequent exercise (1.21, 1.07–1.36), higher income (1.18, 1.06–1.32), urban residence (1.17, 1.06–1.29), and presence of hypothyroidism (1.60, 1.40–1.82) or hyperthyroidism (1.38, 1.19–1.61) were associated with higher thyroid cancer occurrence among women. In conclusion, age ≥ 55 years and current smoking were associated with lower thyroid cancer occurrence, while BMI ≥ 25 kg/m2, higher income, urban residence, hypothyroidism, and hyperthyroidism were associated with higher occurrence in both men and women.

Thyroid and Fertility: Recent Advances

Background: Thyroid disease is prevalent in women of reproductive age, while infertility is common in women with thyroid dysfunction. In this study, we review the recent advances in the field of thyroid and fertility since the publication of the 2017 American Thyroid Association pregnancy guidelines. Summary: Recent studies have confirmed associations of thyrotropin (TSH) elevation and/or thyroid autoimmunity with infertility and low ovarian reserve in subsets of women, and have led to a better understanding of the pathogenesis linking thyroid autoimmunity with infertility. Even though the benefit of treating patients with TSH >4 mIU/L has been confirmed in a large retrospective cohort study, two large randomized controlled trials have failed to show benefit of thyroid hormone on obstetrical outcomes in euthyroid women with thyroid autoimmunity. New data have emerged regarding the potential gonadal toxicity of radioactive iodine (RAI), based on its impact on ovarian reserve and sperm chromosomal abnormalities. Conclusions: There is continued evidence supporting an important role of thyroid hormone in regulation of reproductive tissues at many levels. Recent randomized trials have failed to identify a benefit of thyroid hormone in euthyroid women with thyroid autoimmunity. Further research in the field is needed to more completely delineate the relevant pathways and identify women who may benefit from levothyroxine treatment. The impact of RAI on fertility also merits further investigation.

Longitudinal Analysis of the Effect of Radioiodine Therapy on Ovarian Reserve in Females with Differentiated Thyroid Cancer

Background: Although international guidelines have become more conservative on the use of radioactive iodine (RAI) therapy, it is still one of the cornerstones of the treatment of patients with advanced differentiated thyroid cancer (DTC). As a large proportion of females diagnosed with DTC is in their reproductive years, knowledge about the effect of RAI on their gonadal and reproductive function is important. Earlier studies evaluating Anti-Müllerian hormone (AMH) as a representative of ovarian reserve were either cross-sectional, had relatively low numbers, had no patients with multiple RAI therapies, or had a relatively short follow-up. The primary aim of our study was, therefore, to prospectively evaluate the effect of RAI on AMH in women undergoing treatment for DTC. Methods: We included females, aged 16 years until menopause, who were scheduled to undergo their first RAI treatment for DTC at our hospital. Serum AMH was measured before initial therapy and regularly thereafter. Repeated measurement analysis was used to assess the changes of AMH concentrations over time, and how this is influenced by age and cumulative RAI dose. Results: Longitudinal AMH assessments were available in 65 patients (mean age 32 years, median of five measurements during median follow-up of 34 months). AMH concentrations changed nonlinear over time, decreased until 12 months in the single RAI group (-55%), and stabilized thereafter. In the multiple RAI group, after stabilization, a further decrease occurred (-85% after 48 months). Age in both RAI groups significantly influenced AMH change over time, with younger patients (<35 years of age) showing a less steep decrease. Conclusions: In a population of female DTC patients treated with total thyroidectomy and a single RAI therapy, AMH concentrations significantly dropped during the first year after initial therapy, and thereafter they remained stable. In patients receiving multiple RAI therapies, a further decrease was seen. Age at baseline significantly influenced AMH change over time. These results support a less aggressive treatment with RAI in low-risk patients as is advocated in the current American Thyroid Association (ATA) guidelines, especially in females older than 35 years of age with the desire to have a child.

Association Between Maternal Thyroid Hormones and Birth Weight at Early and Late Pregnancy

CONTEXT

Previous studies suggest that maternal thyroid function affects fetal growth, but the association between combined thyroid hormones from early to late pregnancy and newborn birth weight remains unknown.

OBJECTIVE

To explore the association of maternal thyroid function during early and late pregnancy with birth weight.

DESIGN

A large prospective cohort study of a Chinese population.

SETTING

This study recruited pregnant women who underwent first-trimester prenatal screenings at the International Peace Maternity and Child Health Hospital between January 2013 and December 2016.

PARTICIPANTS

This study enrolled 46,186 mothers in whom TSH, free thyroxine (FT4), T3, and thyroid peroxidase antibody concentrations were measured in the first and third trimesters and in whom data on birth weight were available.

MAIN OUTCOME MEASURES

Birth weight, small for gestational age, large for gestational age (LGA).

RESULTS

A higher TSH or FT4 concentration, or a lower T3 concentration, during the first or third trimester was associated with a lower birth weight. The lowest percentiles of maternal FT4 (FT4 < 2.5th percentile) in both trimesters were associated with a 0.34-SD higher birth weight. The effect estimates were greater in those in the first trimester (0.23 SD) or in the third trimester (0.17 SD). The association of maternal TSH and FT4 with birth weight differed according to fetal sex.

CONCLUSIONS

Persistently low FT4 concentrations throughout pregnancy were associated with higher birth weight and an increased risk of LGA. Based on these findings, we recommend monitoring mildly altered concentrations of thyroid hormone throughout pregnancy.