In contrast , we did not find any beneficial effect of  folate supplementation on placental weight or on length of gestation .
There is a paucity of well-conducted RCTs investigating the effect of  folate supplementation on health outcomes in pregnancy .
The beneficial effect of folic acid supplementation before and shortly after conception is well recognized , whereas the effect of supplementation during the second and third trimesters is controversial and poorly documented .
Our aims were to systematic ally review r and omized controlled trials ( RCTs ) investigating the effect of folate supplementation on birth weight , placental weight and length of gestation and to assess the dose – response relationship between folate intake ( folic acid plus dietary folate ) and health outcomes .
Prevention of the first occurrence of neural-tube defects by periconceptional vitamin supplementation.
BACKGROUND The risk of recurrent neural-tube defects  is decreased in women who take folic acid or multivitamins containing such during the periconceptional period . The extent to which folic acid supplementation can reduce the first occurrence of defects is not known . METHODS We conducted a r and omized , controlled trial of periconceptional multivitamin supplementation to test the efficacy of this treatment in reducing the incidence of a first occurrence of neural-tube defects .  Women planning a pregnancy ( in most cases their first ) were r and omly assigned to receive a single tablet of a vitamin supplement ( containing 12 vitamins , including 0.8 mg of folic acid ; 4 minerals ; and 3 trace elements ) or a trace-element supplement ( containing copper , manganese , zinc , and a very low dose of vitamin C ) daily for at least one month before conception and until the date of the second missed menstrual period or later . RESULTS Pregnancy was confirmed in 4753 women . The outcome of the pregnancy ( whether the fetus or infant had a neural-tube defect or congenital malformation ) was known in 2104 women who received the vitamin supplement and in 2052 who received the trace-element supplement .  Congenital malformations were significantly more prevalent in the group receiving the trace-element supplement than in the vitamin-supplement group ( 22.9 per 1000 vs. 13.3 per 1000 , P = 0.02 ) . There were six cases of neural-tube defects in the group receiving the trace-element supplement , as compared with none in the vitamin-supplement group ( P = 0.029 ) . The prevalence of cleft lip with or without cleft palate was not reduced by periconceptional vitamin supplementation . CONCLUSIONS Periconceptional vitamin use decreases the incidence of a first occurrence of neural-tube defects
In contrast , we did not find any beneficial effect of  folate supplementation on placental weight or on length of gestation .
There is a paucity of well-conducted RCTs investigating the effect of  folate supplementation on health outcomes in pregnancy .
The beneficial effect of folic acid supplementation before and shortly after conception is well recognized , whereas the effect of supplementation during the second and third trimesters is controversial and poorly documented .
Our aims were to systematic ally review r and omized controlled trials ( RCTs ) investigating the effect of folate supplementation on birth weight , placental weight and length of gestation and to assess the dose – response relationship between folate intake ( folic acid plus dietary folate ) and health outcomes .