An abortion is the termination of a pregnancy by the removal or expulsion of an embryo or fetus from the uterus, resulting in or caused by its death. An abortion can occur spontaneously due to complications during pregnancy or can be induced. Abortion as a term most commonly refers to the induced abortion of a human pregnancy, while spontaneous abortions are usually termed miscarriages.

Abortion has a long history and has been induced by various methods including herbal abortifacients, the use of sharpened tools, physical trauma and other traditional methods. Modern medicine utilizes medications and surgical procedures to induce abortion. The legality, prevalence, and cultural views on abortion vary substantially around the world. In many parts of the world there is intense public debate over the ethical and legal aspects of abortion. The approximate number of induced abortions performed worldwide in 2003 was 42 million, which declined from nearly 46 million in 1995.

Spontaneous abortion

main: Miscarriage

Spontaneous abortion (also known as miscarriage) is the expulsion of an embryo or fetus due to accidental trauma or natural causes. Most miscarriages are due to incorrect replication of chromosomes; they can also be caused by environmental factors. Spontaneous abortions, generally referred to as miscarriages, occur when an embryo or fetus is lost due to natural causes before the 20th week of gestation. A pregnancy that ends between 20 and 37 weeks of gestation, if it results in a live-born infant, is known as a "premature birth". When a fetus dies in utero after about 20 weeks, or during delivery, it is termed "stillborn". Premature births and stillbirths are generally not considered to be miscarriages although usage of these terms can sometimes overlap.

Most miscarriages occur very early in pregnancy. Between 10% and 50% of pregnancies end in clinically apparent miscarriage, depending upon the age and health of the pregnant woman. In most cases, they occur so early in the pregnancy that the woman is not even aware that she was pregnant. One study testing hormones for ovulation and pregnancy showed a rate of pregnancy in exposed ovulatory cycles of 59.6%; with 61.9% of conceptuses lost prior to 12 weeks of which 91.7% occuried subclinically, without the knowledge of the mother.

The risk of spontaneous abortion decreases sharply after the 10th week from the last menstrual period (LMP), -Q&A: Miscarriage. (August 6 , 2002). BBC News. Retrieved January 10, 2007.
- Lennart Nilsson. (1990) A Child is Born. with a loss rate between 8.5 weeks LMP and birth of about two percent; pregnancy loss is “virtually complete by the end of the embryonic period."

This risk of spontaneous abortion is greater in those with a known history of several spontaneous abortions or an induced abortion, those with systemic diseases, and those over the age 35. Other causes can be infection (of either the woman or fetus), immune response, or serious systemic disease. A spontaneous abortion can also be caused by accidental trauma; intentional trauma or stress to cause miscarriage is considered induced abortion or feticide.