Chapter 13. Early Pregnancy Risks

Essentials of Diagnosis

• Suprapubic pain, uterine cramping, and/or back pain
• Vaginal bleeding
• Cervical dilation
• Passage of products of conception
• Quantitative β-human chorionic gonadotropin that is falling or not adequately rising
• Abnormal ultrasound findings (eg, empty gestational sac, lack of fetal growth or fetal cardiac activity)

General Considerations

Spontaneous abortion is the most common complication of pregnancy and is defined as the passing of a pregnancy at less than 20 weeks of gestation. It implies the spontaneous loss of an embryo or fetus weighing less than 500 g. Threatened abortion is bleeding arising from within the uterus that occurs before the 20th completed week in a viable pregnancy. The patient may or may not experience pain or cramping; however, there is no passage of products of conception and no cervical dilation. Complete abortion is the expulsion of all of the products of conception before the 20th completed week of gestation, whereas incomplete abortion is the expulsion of some, but not all, of the products of conception. Inevitable abortion refers to bleeding from within the uterus before the 20th week, with dilation of the cervix but without expulsion of the products of conception. The term missed abortion describes a nonviable pregnancy that has been retained in the uterus without cervical dilation and without the spontaneous passage of products of conception. In septic abortion, embryonic or fetal demise has occurred, and intrauterine infection has developed, which has the potential risk of spreading systemically.

Although the true incidence of spontaneous abortion is unknown, approximately 15% of clinically evident pregnancies and up to 50% of chemically evident pregnancies end in spontaneous abortion. Eighty percent of spontaneous abortions occur before 12 weeks' gestation.

The incidence of abortion is influenced by the age of the mother and by a number of pregnancy-related factors, including the number of previous spontaneous abortions, a previous intrauterine fetal demise, and a previous infant born with malformations or known genetic defects. Additionally, chromosomal abnormalities in either parent, such as balanced translocations, and medical comorbidities, such as thyroid disease and diabetes mellitus, may influence the rate of spontaneous abortion.

Pathogenesis

An abnormal karyotype is present in as many as 50% of spontaneous abortions occurring during the first trimester. The incidence decreases to 20–30% of second-trimester losses and to 5–10% of third-trimester losses. The majority of chromosome abnormalities are trisomies (56%), followed by polyploidy (20%) and monosomy X (18%).

Other suspected causes of spontaneous abortion are less common, and these include infection, anatomic defects, endocrine factors, immunologic factors, and exposure to toxic substances. In a significant percentage of spontaneous abortions, the etiology is unknown, even with genetic testing.

Genetic Abnormalities

Aneuploidy, an abnormal chromosomal number, is the most common genetic abnormality, accounting for up to 50% of clinical miscarriages. Monosomy X or Turner's syndrome is the ...