Although we consider miscarriage any loss of a pregnancy that happens before the 23rd week of pregnancy (current limit for considering a viable gestation), the vast majority of them comes about the first trimester of pregnancy.
Miscarriage, possible causes
Among the possible causes of miscarriage are some that have an established relationship and others whose evidence is controversial.
Among the well-established miscarriages causes are genetic, inherited or spontaneous, that causes up to 60-80% of miscarriages according to studies, and some coagulation disorders (thrombophilia) and immunity (antiphospholipid syndrome) who are responsible of the 10-15% of miscarriages, many of them are repetition miscarriages.
Genetic causes
Among the genetic causes, the most common are those that affects the egg (especially in women over 40 years), the sperm (especially in men with important male factors) and parents (karyotype abnormalities such as translocations).
The antiphospholipid syndrome is an immune disorder mediated by antibodies that attack the embryo and alter negatively the implantation and subsequently cause thrombosis at the level of the placenta, which can cause complications during pregnancy.
Finally thrombophilias are disorders of the coagulation processes that cause an excessive clotting. Not all thrombophilia produce miscarriages but there are some like mutations of Factor V Leiden or the factor II prothrombin relating to an increased risk of miscarriage. Today they are also studying the alterations of factor XIII because it seems a very important role in the maintenance of early pregnancy phases.
Other causes
Other causes of miscarriage unproven but very likely are the alterations of the anatomy of the uterus (malformation of the cavity, submucosal fibroids, polyps ...). Finally there are unproven causes but unlikely. Here hormonal alterations (thyroid problems, diabetes, hyperprolactinemia ...) or certain infections (Mycoplasma, Chlamydia ...) are included.
There are factors that have traditionally been considered miscarriage cause that currently are being checked over, like the stress, because the mechanisms that can produce it are unclear.
Nowadays we are studying the possibility that one of the causes of miscarriage is that the woman's body rejects the pregnancy because it considers it a foreign body (which is called alloimmunity). This happens because during the pregnancy a phenomenon called immune tolerance comes off, whereby the woman does not reject the embryo even though it has 50% of its genetic material that belongs to the father. When these control mechanisms fail women's defenses attack the embryo causing miscarriage.