Despite some evidence to the contrary, the folate supplements women are advised to take before getting pregnant do not appear to significantly boost the odds of having twins, according to the results of a large study.

Folate is a B vitamin found in foods such as spinach and other leafy greens, beans and orange juice. Its synthetic form, folic acid, is used in supplements and is added to many breakfast cereals, breads and other grains.

In the new study, of more than 176,000 Norwegian women who gave birth between 1998 and 2001, researchers found no evidence that pre-pregnancy folic acid use increased the chances of a twin pregnancy.

The findings, published in the journal Epidemiology, conflict with some past research, including a Swedish study that linked folic acid use to a 45-percent greater likelihood of having twins.

The new research suggests that those earlier findings are largely explained by the fact that women who undergo in vitro fertilization (IVF) are more likely to both take folic acid supplements and to have twins.

In the U.S. and other countries in North and South America, health authorities require that many grain products be fortified with folic acid, due to evidence that the B vitamin reduces a woman’s risk of having a baby with a neural tube defect - a severe malformation of the spine or brain.

These birth defects, which include spina bifida, take shape very early in pregnancy, often before a woman knows she is pregnant. Because of this, health experts in the U.S. advise that all women of childbearing age get 400 micrograms of folate each day through food and supplements, increasing to 600 micrograms during pregnancy.

But folate fortification is not the universal norm. The idea is controversial in Sweden, the U.K. and other European countries, with the supposed link to twin pregnancy being an argument against folic acid supplementation.

The new findings could help sway such policies, according to study leader Dr. Stein Emil Vollset, of the University of Bergen in Norway.

Vollset and his colleagues obtained their findings from data in Norway’s national birth registry, which includes information on IVF pregnancies and the use of multivitamins and folic acid supplements before and during pregnancy. The women gave birth between December1998 and December 2001.

The researchers found that overall, women who took folic acid before pregnancy were 59 percent more likely to have twins than women who did not take the supplements.

However, the link was nearly erased when the investigators looked only at women who conceived naturally.

Women who undergo assisted reproduction are much more likely than other women to have multiple births, because they typically have two or more embryos implanted in the uterus during the procedure.

In this study, they were also much more likely to take folic acid before getting pregnant. About one quarter of women who conceived through IVF took pre-pregnancy folic acid, versus only six percent of those who conceived naturally.

The results, Vollset told Reuters Health, suggest that the link between folate and twin pregnancies found in earlier studies is largely explained by IVF patients’ greater use of folic acid.

Still, it’s known that some twin pregnancies are never recognized because one twin is lost very early after conception. Some have speculated, Vollset said, that folic acid or other vitamins may improve the chances of both fetuses surviving.

In fact, he and his colleagues found that women who used multivitamins during pregnancy - though not before - were slightly more likely to have twins, even after accounting for IVF pregnancies.

This could mean that the vitamins improved the survival odds of both twins, according to the researchers. On the other hand, they add, women who find out they’re having twins may simply be more likely to take vitamin supplements.

SOURCE: Epidemiology, March 2005.