Folic acid supplementation is well known to reduce the risk of neural tube defects to a developing fetus, but a new study says hopeful moms-to-be should start the process even before they get pregnant to reduce the risk of low birth weight.

Babies considered "small for gestational age" (SGA) represent the smallest 10 per cent of babies born.

SGA babies run the risk of contracting diseases later on including diabetes, hypertension, obesity, cardiovascular disease and mental health problems, according to the researchers of the UK-based study.

Although the benefits of pre-conceptual folic acid supplementation are nothing new and it's recommended for women in the U.K., the researchers note that the majority of women don't follow the recommendation.

Participants in the study had a mean maternal age of 28.7 and a median body mass index (BMI) of a healthy 24.7. Nearly 82 per cent were non-smokers and 42 per cent were first-time mothers.

Nearly 85 per cent of the mothers took folic acid during their pregnancies; however, of the pool of 108,525 pregnancies, the time at which they began supplementation was recorded in just 39,416 cases. Of those, 25.5 per cent had started supplementation before conceiving.

Birth weights in the lowest 10 and 5 per cent represented 13.4 per cent and 7 per cent of the total sample. Among women who hadn't taken folic acid, those percentages were 16.3 and 8.9, respectively. Among mothers who had taken folic acid before conception, the percentages were the lowest: 9.9 and 4.8.

In cases when the mother started supplementation after becoming pregnant, 13.8 per cent of babies were in the bottom 10 per cent and 7.1 in the bottom 5 per cent.