Women who are too thin can affect pregnancy. When a woman is too thin with a body mass index less than 18, it may cause ovarian failure to ovulate and amenorrhea, which can affect pregnancy. The normal female body mass index is between 18 and 24, and anything above this range may affect pregnancy. If the body mass index is less than 18, you should enhance nutrition, eat more high-quality protein, such as fish, meat, eggs and milk, and also eat more fresh vegetables and fruits. At the same time, physical exercise should be carried out to enhance physical fitness and increase body mass. However, it is important to note that the body mass index should not exceed 30. If the body mass index exceeds 30, ovarian failure to ovulate will also affect pregnancy, and it is best to keep the body mass index between 18 and 24.