Selenium (Se) is an essential element for humans, but an excessive intake can also produce toxic effects. The association between Se exposure and fetal growth is currently unclear.

The study is conducted to investigate the associations between exposure to Se during pregnancy and head circumference of fetus in the third trimester.

All participants were selected from the Prenatal Environment and Offspring Health (PEOH) cohort conducted since 2016 in Guangzhou, China. They completed face-to-face interviews at baseline in their first trimester (≤ 13 gestational weeks) and at follow-up visits in their third trimester (≥ 28 gestational weeks). Their prenatal records were retrieved from the hospital information system, and 15.0 mL spot urine samples were collected in the first and the third trimesters respectively. Fetal head circumference was measured by ultrasound examination in the third trimester. A total of 2739 pregnant women, including 2138 and 1383 participants in the first trimester and the third trimester respectively, were finally included to estimate the associations between fetal head circumference in the third trimester with Se exposure in the first and the third trimesters, respectively. The urinary Se concentrations (ω_Se) were corrected by creatinine (μg·g^-1, thereafter) and transformed by natural logarithm (lnω_Se). After adjusting the maternal age, education, family income, pre-pregnancy body mass index, gravidity, gestational weeks, fetal sex, second-hand smoke exposure, calcium intake, and meat and seafood intake, multiple linear regression models were applied to estimate the overall and sex-specific associations between Se exposure level and fetal head circumference.

Among the 2 739 pregnant women, there were 1 926 pregnant women aged ≥ 30 years (70.3%), 1 474 male fetuses (53.8%), 2 313 pregnant women with high school education and above (84.4%), 871 pregnant women exposed to passive smoke during pregnancy (31.8%), and 2 637 pregnant women with gestational age ≥ 37 weeks (95.6%). The median (P₂₅, P₇₅) of urinary Se level was 38.5 (30.1, 44.5) μg·g^-1 in the first trimester and 38.9 (28.7, 54.0) μg·g^-1 in the thrid trimester, respectively. The head circumference of fetuses in the third trimester was (218.1±41.4) mm. According to the results of multiple linear regression models, no association was found between the Se level in the first trimester and the fetal head circumference in the third trimester. For each unit increase of urinary lnω_Se in the third trimester, the decrease in fetal head circumference and its 95% confidence interval (CI) were 7.04 (3.41-10.67) mm (P < 0.001). The participants were divided into four groups (Q1-Q4) according to the quartiles of lnω_Se in the third trimester. Compared with the Q1 group, the head circumference of fetuses in the Q4 group decreased by 19.47 (12.50-26.43) mm (P < 0.001). In the third trimester of pregnancy, for each unit increase in lnω_Se, the head circumference of male and female fetuses decreased by 5.06 (0.48-9.63) mm (P=0.031) and 11.30 (5.32-17.27) mm (P < 0.001) respectively, and the difference between male and female fetuses was 6.24 (1.69-10.79) mm; compared with the Q1 group, the head circumference of male and female fetuses in the Q4 group was reduced by 18.13 (8.45-27.81) mm (P < 0.001) and 21.26 (11.23-31.30) mm (P < 0.001) respectively.

Under the exposure level of the selected population, a higher urinary Se level in the third trimester is associated to a reduction of fetal head circumference in the third trimester.