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Spontaneous abortion occurs in 15% of diagnosed pregnancies in women who are 15-44 years of age. Recurrent spontaneous abortions occur in about 3-4% of these women, defined as the loss of three or more consecutive pregnancies. Most spontaneous abortions occur because of abnormalities in the fetus, causing miscarriage or habitual abortion. Almost 70% of first-trimester abortuses, 30% of second-trimester abortuses, and 3% of stillbirths have abnormal chromosome numbers.
Recurrence Risk
The risk of pregnancy loss increases from 15-20% in the first pregnancy to 40% after one spontaneous abortion, but does not increase thereafter. If a woman has at least one liveborn child, the risk of her first spontaneous abortion is 15%. The prevalence of spontaneous abortion increases with increasing maternal age, although not with gravidity. The risk begins to increase rapidly at age 35 years. The risk of spontaneous abortion at age 40 years is approximately twice that at age 20 years. Although one might suspect that the reason for this increase is the known rise in aneuploid conception with increasing maternal age, this does not wholly account for the rapid rise in spontaneous abortion. Indeed, more euploid pregnancies are lost with increasing maternal age. This fact suggests that other factors, such as decreased uterine blood supply, chronic infections, or deficient luteal phase, may be involved. Furthermore, the endometrial lining contains glycoproteins and integrins important in implantation that may change with age.
Etiology
Genetics
Patients having a karyotypically abnormal abortus are more likely to have another abortus that is abnormal. Conversely, if the first abortus has a normal karyotype.
Anatomic Anomalies
Müllerian Fusion Defects
Müllerian defects of all types are associated with a higher incidence of pregnancy loss. The septate uterus, the most common anatomic abnormality, carries a risk of 70% spontaneous abortion in the first trimester. This is believed to be due to the relatively less vascular nature.
Intrauterine Synechiae
Intrauterine synechiae (Asherman's syndrome) may cause oligomenorrhea, infertility, or spontaneous abortion. Adhesions may follow overzealous curettage of the uterus during the postpartum period, intrauterine surgery (eg, myomectomy), or endometritis.
Leiomyomas
Uterine leiomyomas are usually multiple and may contribute to pregnancy loss, but the pathophysiology is unknown. Location, rather than size, of the leiomyoma is probably the most important factor. Submucous leiomyomas may result in fetal loss through several theoretical mechanisms: 1) endometrial thinning over the surface of the myomas.
Endocrine Abnormalities
Thyroid Disease
The theory that thyroid disorders cause spontaneous abortion is now disproved. Although there is no need to screen asymptomatic patients with spontaneous losses for thyroid disease.
Diabetes Mellitus
When inadequately controlled, type 1 diabetes (formerly referred to as insulin-dependent diabetes mellitus) may increase the risk of spontaneous abortion.
Luteal-Phase Defect
The luteal-phase defect is presumably responsible for abnormal development of the endometrium required for implantation and placentation. The luteal-phase defect results either from a deficient secretion of progesterone or a poor endometrial response to adequate levels.
Infectious Agents
Almost every infectious agent except HIV has been implicated in causing recurrent spontaneous abortion. However, prospective controlled studies are lacking. The only organism for which prospective data are available is T-strain mycoplasma (Ureaplasma urealyticum).