Maternal Thyroid Function and Birth Weight in Twins

Area of Science:

• Reproductive Endocrinology
• Maternal-Fetal Medicine
• Perinatology

Background:

• Thyroid hormones are crucial for fetal development, with twin pregnancies requiring higher levels.
• Associations between thyroid hormones and birth weight are established in singleton pregnancies.
• The specific role of thyroid hormones in twin birth weight and discordance requires further investigation.

Purpose of the Study:

• To investigate the association between thyroid hormones and birth weight in twin pregnancies.
• To explore how fetal sex and chorionicity influence the relationship between thyroid hormones and twin birth weight.
• To identify potential implications for managing twin pregnancies and preventing birth weight discordance.

Main Methods:

• Retrospective cohort study of 1626 pregnant women delivering live-born twins in China (2014-2019).
• Utilized linear mixed models with restricted cubic splines and logistic regression.
• Analyzed associations of thyrotropin (TSH) and free thyroxine (FT4) with birth weight and birth weight discordance.

Main Results:

• Overall, TSH and FT4 were not associated with birth weight in twins.
• Nonlinear associations between FT4 levels and birth weight were observed in male fetuses and dichorionic twins.
• Lower FT4 levels (below 10th percentile) were associated with a higher risk of birth weight discordance (OR=1.58, 95% CI: 1.05-2.33).

Conclusions:

• FT4, not TSH, is associated with birth weight and discordance in twins, influenced by fetal sex and chorionicity.
• These findings highlight the importance of monitoring FT4 levels in twin pregnancies.
• Optimal FT4 management may help prevent birth weight discordance in twin gestations.