Short answer: At least 1 month before trying to conceive, according to CDC, and ideally as a daily habit already. CDC recommends 400 mcg of folic acid a day for anyone who could become pregnant, because neural tube defects (problems of the baby's brain and spine) form in the first weeks, often before a pregnancy is known. After a past pregnancy with a neural tube defect, CDC describes a higher 4,000 mcg amount, set with a doctor.
Folic acid is the form of folate (vitamin B9) used in supplements and fortified foods. CDC says it is the only form of folate shown in studies to help prevent neural tube defects (NTDs) such as spina bifida and anencephaly. The NIH Office of Dietary Supplements notes these defects arise when the neural tube fails to close on days 21 to 28 after conception.
Because that window is so early, CDC says that by the time someone realizes they are pregnant it may be too late to prevent an NTD. That is why the advice is to have folic acid on board before conception, not to start at a positive test.
| Situation | Amount | Timing |
|---|---|---|
| Could become pregnant | 400 mcg a day | Daily, starting at least 1 month before conception and through pregnancy |
| Past pregnancy affected by an NTD, planning again | 4,000 mcg a day | Talk with a doctor first; start 1 month before and continue through the first 3 months |
CDC says the 400 mcg advice comes from the US Public Health Service and is supported by the Institute of Medicine and the US Preventive Services Task Force. It rests on two large randomized trials from the 1990s, one in women who had a previous NTD pregnancy and one in women who had not, which both showed fewer NTDs with folic acid before and during early pregnancy. ODS reports NTD rates in the US fell by 28% after folic acid was added to grain foods in 1998.
A cycle on Clomid, letrozole or injected gonadotropins is a planned attempt at pregnancy, so the CDC advice applies from before the first cycle. Any amount above 400 mcg is a decision for the doctor; CDC notes a doctor might recommend more for some health conditions.
ODS lists an adult upper limit of 1,000 mcg a day for folate from supplements and fortified foods; folate found naturally in food is not part of that limit. The folate page covers the vitamin B12 question.
CDC suggests checking vitamin and fortified food labels to make sure they contain folic acid, and eating folate-rich foods like leafy greens, beans and oranges. Dose Track can set a daily reminder for a prenatal vitamin next to fertility medicines, so the start date is on record for the clinic.
Read more on Dose Track: Folate · Female fertility · Clomid dosing schedule · Ovulation induction