Background Epidemiological studies have revealed that prenatal thallium exposure can lead to adverse birth outcomes.

Objective This study is designed to explore the effects of prenatal thallium exposure on fetal growth and birth outcomes, and assess the critical window period for the health effects of thallium exposure.

Methods A total of 146 pregnant women and their children participated in this study, who were recruited to the Shanghai Maternal-Child Pairs Cohort (MCPC). Information of demographic characteristics and mother and child health was collected by self-designed questionnaires. Inductively coupled plasma mass spectrometry (ICP-MS) was used to detect the concentration of thallium in maternal blood in different gestational periods and cord blood of newborns. A mixed-effect model was used to explore the effect of thallium exposure during pregnancy on fetal growth and development (such as double top diameter, head circumference, abdominal circumference, femur length, and humerus length), and a linear regression model was further used to explore the critical window period for the health effects of thallium exposure.

Results The positive rate of thallium in blood samples was more than 99.0%. The median (P₂₅, P₇₅) values of thallium in maternal blood were 38.7 (32.3, 50.1), 44.3 (34.3, 56.7), and 45.0 (34.9, 54.6) ng·L^-1 in the first, second, and third trimesters of pregnancy, respectively. The thallium concentrations in maternal blood during whole pregnancy were 42.7 (34.2, 53.3) ng·L^-1, and 23.8 (17.2, 35.0) ng·L^-1 in umbilical cord blood. After adjusting for confounding factors, the results of mixed-effect model showed that the exposure level of thallium during pregnancy was negatively correlated with fetal double top diameter (b=-0.91, 95% CI: -1.78--0.02), and not correlated with the other fetal growth indicators (P>0.05); the results were consistent after excluding the influence of lead, cadmium, manganese, and calcium. After thallium concentration was grouped by quartiles, it was found that when thallium concentration was increased by one quartile in the first trimester, the fetal double top diameter, head circumference, femur length, and humerus length were reduced by 3.0, 10.9, 2.6, and 3.1 mm, and the head circumference and abdominal circumference of the fetuses were increased by 8.3 and 16.3 mm, respectively; the fetal abdominal circumference and femur length were increased by 7.3 and 2.1 mm for each quartile increase of thallium concentration in blood in the third trimester, respectively; each quartile increase in thallium concentration in umbilical cord blood resulted in an increase in birth body length of 0.07 mm, but was not associated with other indicators (P>0.05).

Conclusion Thallium exposure during pregnancy has a negative effect on intrauterine growth and development and birth outcomes of the fetuses. The first trimester might be the critical exposure window for thallium to affect fetal growth and development.