Excess weight can cause high blood pressure or diabetes. It also puts a strain on the heart. This strain becomes even greater during pregnancy, when your heart has to work harder to supply blood to you and your baby. Women who are obese have a greater chance of problems during pregnancy, which poses risks for the baby as well as the mother. They also are at increased risk for a condition called macrosomia, in which a fetus grows too large and may have trouble fitting through the birth canal.
Being underweight can also lead to trouble getting pregnant. It also may increase your chances of giving birth to a low birth weight baby. These babies are not easier to deliver and often have problems during labor and after birth.

Infections passed through sexual contact—sexually transmitted diseases (STDs)—also are harmful during pregnancy. Many types of STDs may affect your ability to become pregnant. They also may infect and harm your baby. If you think you or your partner may have an STD, get tested and treated right away. Your partner also may need to be treated. Neither of you should have sex until you have both finished treatment. Infection with human immunodeficiency virus (HIV) can cause harm to mother and baby. Early treatment may help prevent the infection from being passed to the fetus. Because of this, your doctor will offer HIV testing, even if you have no symptoms.