Maternal serum alpha-feto protein (MSAFP) screening - A prenatal blood test that checks for elevated levels of alpha-feto protein, which can indicate a higher chance of serious neural tube defects. MSAFP also screens for the probability of Downs syndrome. Ultrasound and aminocentesis improve the tests' accuracy.

Macrosomia - A condition common to diabetic mothers in which the fetus grows very large.

Mask of pregnancy - See chloasma.

Meconium - The greenish substance that develops in the bowels of a growing fetus and is usually discharged shortly after birth.

Miscarriage - The spontaneous loss of a pregnancy before 20 weeks gestation.

Molar pregnancy - (gestational trophoblastic disease) An abnormal condition of pregnancy characterized by the growth of cysts on the placenta and bleeding during early and middle pregnancy.

Morning sickness - Nausea and vomiting during pregnancy, especially during the first trimester.

Mucus plug - The mass of thick, sticky mucus membranes that accumulates in the cervix during pregnancy.

Multiple pregnancy - A pregnancy with two or more fetuses.