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 neural tube defects: Results of the Medical Research Council Vitamin Study
A r and omised double-blind prevention trial with a factorial design was conducted at 33 centres in seven countries to determine whether supplementation with folic acid ( one of the vitamins in the B group ) or a mixture of seven other vitamins ( A , D , B1,B2,B6,C and nicotinamide ) around the time of conception can prevent neural tube defects ( anencephaly , spina bifida , encephalocele ) . A total of 1817 women at high risk of having a pregnancy with a neural tube defect , because of a previous affected pregnancy , were allocated at r and om to one of four groups -- namely , folic acid , other vitamins , both , or neither .  1195 had a completed pregnancy in which the fetus or infant was known to have or not have a neural tube defect ; 27 of these had a known neural tube defect , 6 in the folic acid groups and 21 in the two other groups , a 72 % protective effect ( relative risk 0.28 , 95 % confidence interval 0.12 - 0.71 ) . The other vitamins showed no significant protective effect ( relative risk 0.80 , 95 % Cl 0.32 - 1.72 ) . There was no demonstrable harm from the folic acid supplementation , though the ability of the study to detect rare or slight adverse effects was limited . Folic acid supplementation starting before pregnancy can now be firmly recommended for all women who have had an affected pregnancy , and public health measures should be taken to ensure that the diet of all women who may bear children contains an adequate amount of folic acid
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 .