A baby’s neural tube starts developing in the first 4 weeks of pregnancy (before your first missed period). So it’s best to start taking folic acid as soon as you start trying to get pregnant. In the UK and Ireland in the 1980s, before women were advised to take folic acid supplements, about 1 in 200 pregnancies resulted in a baby with a neural tube defect. Taking folic acid in early pregnancy reduces this risk to 1 in every 400.