Zahra Panahi - Department of Obstetrics and Gynecology, Vali-e Asr Hospital, Imam Khomeini Hospital Complex, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Seyedeh Noushin Ghalandarpoor-Attar - Baqiyatallah Hospital, School of Medicine, Baqyiatallah University of Medical Sciences, Tehran, Iran
Azade Shabani - Department of Obstetrics and Gynecology, Shahid Taleghani Hospital, School of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran
Mamak Shariat - Fetal and Neonatal Research Center, Vali-e Asr Hospital, Imam Khomeini Hospital Complex, Tehran University of Medical Sciences, Tehran, Iran
Fahimeh Ghotbizadeh - Department of Obstetrics and Gynecology, Vali-e Asr Hospital, Imam Khomeini Hospital Complex, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Sedigheh Hantoushzadeh - Department of Obstetrics and Gynecology, Vali-e Asr Hospital, Imam Khomeini Hospital Complex, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Elham Feizabad - Department of Gynecology, Yas Hospital, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran
Seyedeh Mojgan Ghalandarpoor-Attar - Department of Obstetrics and Gynecology, Baharloo Hospital, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran

Background & Objective: 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. Materials & Methods: We performed a cross-sectional study of ultrasound measurements of the fetal thymus at ۱۸-۲۲ weeks of gestational age in ۹۴ pregnant women and its correlation with maternal serum vitamin D levels from May to July ۲۰۲۱ at the tertiary center of Imam Khomeini Hospital in Tehran, Iran. Results: The mean values of thymus perimeter, thymus-thoracic ratio, thymus transverse diameter, and thymus area in all participants were ۴.۱۸±۰.۵۶ cm, ۰.۳۷±۰.۰۴, ۱.۵۶±۰.۲۱ cm, and ۱.۱۱±۰.۷۶ cm۲, 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. Conclusion: 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.

Pregnancy, Fetal Thymus, Thymus indices, Thymus Size, Vitamin D Deficiency