This study is a randomized placebo-controlled trial of oral weekly vitamin D3 (cholecalciferol) supplementation during the third trimester of pregnancy among women in Dhaka, Bangladesh. The overall goal of the study is to establish whether there is evidence that improving vitamin D status among pregnant women in Bangladesh will enhance the resistance of the infant offspring to infection.

The aims of the study are to assess the effect of supplementation on 1) maternal and infant vitamin D status (based on blood concentrations of a vitamin D metabolite) and, 2) markers of neonatal immune function.

The primary aims of this study are:

AIM #1 - To assess the effect of weekly antenatal administration of oral vitamin D3 (875 mcg/week = 35,000 IU week ≈ 5,000 IU per day) started in the third trimester (26-29 weeks gestation) on maternal vitamin D status and fetal-neonatal vitamin D status (cord blood), in comparison to a placebo control supplement.

AIM #2 - To demonstrate the maternal and fetal safety of weekly maternal antenatal (second and third-trimester) vitamin D supplementation at a dose of 875 mcg/week by monitoring maternal serum calcium, urinary calcium excretion, cord blood calcium concentration, and newborn clinical parameters.

AIM #3 - To measure the effect of antenatal vitamin D supplementation on selected biomarkers of fetal-neonatal immune function in cord blood: in vitro stimulated cord blood mononuclear cell (CBMC) LL-37 expression, gene expression related to inflammatory and immunoregulatory pathways, Th1/Th2 cytokine secretion, and bactericidal properties.

Inclusion Criteria:

• Women aged 18 to <35 years.
• Current residence in Dhaka at a fixed address
• Plans to have the delivery performed at the Shimantik maternity center, and to stay in Dhaka throughout the pregnancy and for at least one month past the date of delivery.
• Gestational age of 26th to 29th (inclusive), estimated based on the first day of the last menstrual period (LMP).

Exclusion Criteria:

• Use of any dietary supplement containing more than 400 IU/day (10 mcg/day) of vitamin D within the month prior to enrolment, or refusal to stop taking supplemental vitamin D at any dose after enrollment.
• Current use of anti-convulsant or anti-mycobacterial (tuberculosis) medications.
• Severe anemia (hemoglobin concentration < 70 g/L).
• Complicated medical or obstetric history that may increase the risk of preterm birth or labor/delivery complications, based on self-report or clinical assessment by physician (e.g., cardiovascular disease, uterine hemorrhage, placenta previa, threatened abortion, hypertension, preeclampsia, preterm labor, or multiple gestation).
• Prior history of delivery of an infant with a major congenital anomaly, birth asphyxia, or perinatal death (stillbirth or death within first week of life).