Relationship Between Thyroid Function Tests and Birth Parameters at 41-Week-And-Above Pregnancies: A Prospective Cohort Study.

Background: In the literature, there is no study investigating the relationship between thyroid hormones in pregnancies at 41 weeks and above and the birth timing, labor duration, frequency of fetal distress, premature rupture of membranes (PROM), and maternal hemogram values. Methods: A total of 68 nulliparous pregnant women who were admitted to Basaksehir Cam and Sakura City Hospital with indications of delivery between August 2023 and January 2024, between the ages of 20 and 38 and with no comorbidities, were included in the study. Pregnant women with ≥41 weeks of gestation were classified as the late-term pregnancy group (n = 37), and those between 37 and 38 weeks were classified as the control group (n = 31). The thyrotropin (TSH), free thyroxine (FT4), and hemoglobin levels and relevant parameters were evaluated. Results: The FT4 values of pregnant women diagnosed with fetal distress in the entire population were observed to be statistically significantly lower (p < 0.05). A statistically significant negative linear relationship was detected between the FT4 values of the entire population and the weights of newborns (p < 0.05). It was determined that, as the FT4 values decreased, the newborn weights increased. There was no statistically significant difference between the two groups in terms of the TSH/FT4 values, birth types, labor duration, or postpartum Hb/Htc decrease (p > 0.05). No statistically significant relationship was found between the TSH/FT4 values of the entire population and the diagnosis of PROM, labor duration, or Hb/Htc decrease amount (p > 0.05). Conclusions: TSH/FT4 levels may be important in the mature and late-mature periods of pregnancy. There may be an association between the FT4 levels and the fetal distress risk, type of birth, and newborn weight.