Abortion is the spontaneous or induced (therapeutic) expulsion of the products of conception from the uterus before 20 weeks gestation (fetal weight less than 500 g [17½2 oz]). Up to 15% of all pregnancies and about 30% of all first pregnancies end in spontaneous abortion (miscarriage). At least 75% of miscarriages occur during the first trimester.
Spontaneous abortion may result from fetal, placental, or maternal factors. (See Types of spontaneous abortion.) Fetal factors usually cause abortions before the 12th week of gestation and include:
defective embryologic development resulting from abnormal chromosome division (most common cause of fetal death)
faulty implantation of the fertilized ovum
failure of the endometrium to accept the fertilized ovum.
Types of spontaneous abortion
Threatened abortion: Bloody vaginal discharge occurs during the first half of pregnancy. About 20% of pregnant women have vaginal spotting or actual bleeding early in pregnancy; of these, about 50% abort.
Inevitable abortion: Membranes rupture or the cervix dilates
or both. As labor continues, the uterus expels the products of conception.
Incomplete abortion: Uterus retains part or all of the placenta. Because part of the placenta may adhere to the uterine wall, bleeding continues.
Complete abortion: Uterus passes all the products of conception.
Missed abortion: Uterus retains the products of conception for 2 months or more after the death of the fetus. Uterine growth ceases; uterine size may even seem to decrease. Prolonged retention of the dead products of conception may cause coagulation defects such as disseminated intravascular coagulation.
Habitual abortion: Spontaneous losses of three or more consecutive pregnancies constitute habitual abortion.
Septic abortion: Infection accompanies abortion.
Placental factors usually cause abortion around the 14th week of gestation, when the placenta takes over the hormone production necessary to maintain the pregnancy. These factors include:
premature separation of the normally implanted placenta
abnormal placental implantation.
Maternal factors usually cause abortion during the second trimester and include: