Folic acid, also known as vitamin B9, is a water-soluble vitamin that was originally extracted from spinach leaves, hence the name “folic acid”. Brown vaginal discharge is a manifestation of abnormal embryonic or fetal development in early pregnancy (e.g. stillbirth, miscarriage) and is affected by many unfavorable factors, of which folic acid deficiency is only one. Folic acid plays an important role in the synthesis of proteins and nucleic acids and the metabolism of various amino acids. If it is severely deficient in early pregnancy (mostly in the 8th-9th weeks of pregnancy), it will reduce homocysteine metabolism, and high concentrations of homocysteine may cause placental vascular damage, affecting the blood supply from the placenta to the fetus, causing stillbirth and miscarriage, which may manifest as vaginal brown discharge. Folic acid can prevent neurological defects (mainly anencephaly, spina bifida and brain bulge), tumors and cardiovascular diseases. Folic acid supplementation from the preparation period to the third trimester is effective in preventing neurological deformities, but in the middle and late stages of pregnancy, fetal DNA synthesis, placenta, maternal tissues and increased red blood cells all require significantly more folic acid, and deficiency can increase the risk of anemia and hypertension, so it is recommended to supplement folic acid throughout pregnancy. Folic acid cannot be synthesized in the body and can only be ingested exogenously, such as dark green vegetables, citrus fruits, beans, nuts and animal liver are rich in natural folic acid, but the human body cannot absorb that much by food intake alone, so it is necessary to take folic acid tablets daily for at least 4 weeks to effectively improve the folic acid deficiency in the body and reach the preventive concentration (0.4mg daily for 6-8 weeks; 0.8mg daily for 4 weeks). Therefore, folic acid supplementation should start from the preparation of pregnancy. Folic acid deficiency in early pregnancy increases the risk of miscarriage, stillbirth and neurological defects, while folic acid deficiency in middle and late pregnancy increases the risk of anemia and hypertension, so it is recommended to start folic acid supplementation from 3 months before pregnancy at 0.4-0.8mg daily for the whole pregnancy.