High-risk pregnancies may include patients who are older than 35 and younger than 17, patients who are underweight or overweight, pregnant with more than one fetus, patients who have gone into premature labor, babies with birth defects, and patients with high blood pressure, heart disease, diabetes, or other medical problems. Obstetricians who practice maternal-fetal medicine are also known as perinatologists. Perinatologists who perform fetal surgery are known as fetal surgeons, or maternal-fetal surgeons.