Recurrent pregnancy loss is classically defined as the occurrence of three or more consecutive pregnancy loss; however, the American Society of Reproductive Medicine (ASRM) has recently redefined recurrent pregnancy loss as two or more pregnancy losses. A pregnancy loss is defined as a clinically-recognized pregnancy involuntarily ending before 20 weeks. A clinically-recognized pregnancy means that the pregnancy has been visualized on an ultrasound or that pregnancy tissue was identified after a pregnancy loss.

What Causes Recurrent Pregnancy Loss ?

Most pregnancy losses result from chromosomal, or genetic, abnormalities, and are random events. The abnormality may come from the egg, the sperm, or the early embryo. Approximately 12-15% of all clinically recognized pregnancies end in miscarriage; however, it is estimated that at least 30-60% of all conceptions will end within the first 12 weeks of gestation. Up to 50% of the time, the woman doesn’t even realize that she was ever pregnant. The risk of miscarriage increases with the number of previous pregnancy losses, but is typically less than 50%.

Advancing maternal age is associated with an increased risk of miscarriage, which is thought to be due to poor egg quality leading to chromosomal (genetic) abnormalities. Sometimes, the mother or father themselves may have a slight irregularity in their genes, but the offspring could be more severely affected and thus result in miscarriage.

Sometimes, there could be an abnormality in the uterus (the womb) that leads to miscarriage. The miscarriage may be due to poor blood supply to the pregnancy or inflammation. Some women may be born with an irregularly shaped uterus, and some women may develop abnormalities with their uterus over time.

A woman’s immune system may also play a role in recurrent pregnancy loss. Hormone abnormalities may also impact pregnancy loss, including thyroid disease and diabetes. Abnormalities in a mother’s blood clotting may also affect pregnancy loss.

Generally speaking, environmental factors, stress, and occupational factors do not seem to be related to pregnancy loss.

Source
https://www.uclahealth.org/obgyn/recurrent-pregnancy-loss