Maternal Vitamin D Concentration in Mid-pregnancy and Its Effect on Fetal Thymus Size: A Report from a Tertiary Center in Iran

 The thymus gland significantly affects fetal immune system maturation. Additionally, there is a linear association between thymus gland size and its performance. Given the high prevalence of vitamin D deficiency in Iran and scarce studies with conflicting results, subjecting maternal vitamin D concentration effect on fetal thymus, we decided to investigate maternal vitamin D concentration and its relation to fetal thymus size in mid-gestation. This study also aimed to generate a race-specific reference range.

We performed a cross-sectional study of ultrasound measurements of the fetal thymus at 18-22 weeks of gestational age in 94 pregnant women and its correlation with maternal serum vitamin D levels from May to July 2021 at the tertiary center of Imam Khomeini Hospital in Tehran, Iran.

The mean values of thymus perimeter, thymus-thoracic ratio, thymus transverse diameter, and thymus area in all participants were 4.18±0.56 cm, 0.37±0.04, 1.56±0.21 cm, and 1.11±0.76 cm2, respectively. There was a trend toward decreased thymus perimeter and transverse thymus diameter with decreasing level of maternal vitamin D. There was also a significant correlation between thymus perimeter and transverse thymus diameter with fetal biometric indices and gestational age. Furthermore, a significant correlation was observed between the thymus perimeter and transverse thymus diameter.

We generated a race-specific nomogram for fetal thymus size in Iranian pregnant women. Moreover, the observed trend toward decreased fetal thymus size with decreasing maternal vitamin D levels requires further prospective investigations. A high prevalence of vitamin D deficiency and low compliance with daily vitamin D intake during pregnancy was also shown, which requires a solution.