Previous studies have linked low maternal levels of vitamin C with an increased risk of PROM, but it is not known whether supplementation could help reduce this risk. Researchers from Mexico's National Institute of Perinatology investigated the effects of vitamin C supplements. They recruited 120 women who were halfway through their pregnancy. Half were given 100 mg vitamin C per day and the other half received a placebo until week 36 of pregnancy. Vitamin C concentrations were measured in plasma and leukocytes (white blood cells) every four weeks. All of the women's plasma levels of vitamin C dropped during the study, but leukocyte levels of vitamin C, a measure of stored vitamin C, increased in the supplement group. PROM occurred in 25 per cent of pregnancies in the placebo group and 8 per cent in the supplement group. This equates to a protective effect of 74 per cent. However, supplementation did not affect the incidence of preterm delivery.

The researchers conclude that daily supplementation with 100 mg vitamin C after 20 weeks of gestation is effective at reducing the incidence of PROM. Because PROM may be responsible for 40 per cent or more of all cases of preterm delivery, vitamin C supplements may be a valuable tool in sustaining pregnancy to term, they report. They add that it would be worthwhile to develop educational programs to promote adequate intakes of vitamin C through diet rather than reliance on supplements during pregnancy, and suggest that it might be necessary to adjust the current recommended dietary allowance for pregnant women.
Casanueva, E et al. Vitamin C supplementation to prevent premature rupture of the chorioamniotic membranes: a randomized trial. American Journal of Clinical Nutrition, Vol. 81, April 2005, pp. 859-863

Editor’s comment: Extensive research has shown that it takes at least 400 mg/day of vitamin C to saturate blood plasma. It is possible that the incidence of PROM could be even further reduced by supplementing with higher doses.