Part of “CHAPTER 107 – CONCEPTION, IMPLANTATION, AND EARLY DEVELOPMENT“

The most common manifestation of the failure of embryonic and fetal development is spontaneous abortion: the failure of conception to produce a live birth. Spontaneous abortion, then, is either the disruption of pregnancy once it can be recognized or the expulsion of a nonviable fetus. Precise clinical definitions are much more difficult. Most often, these definitions must invoke low birth weight, because below certain weights, the fetus is unlikely to survive. Other definitions include loss of pregnancy before 20 or 28 weeks of gestation. Accurate estimates of the incidence of spontaneous abortion, therefore, are difficult to obtain. The frequency of clinically evident spontaneous abortion is ˜15% of pregnancies. Undoubtedly, the risk is much higher in women with a previous spontaneous abortion, with the risk as high as 46% after three consecutive abortions. However, if the abortus is karyotypically abnormal, the risk of consecutive abortion is substantially lower.69,70 The association of prior spontaneous abortion with subsequent poor pregnancy outcome has been well documented, even when all other risk factors have been controlled. The effects of specific risk factors seem to be much stronger than the history.

Early pregnancy losses are occult. Early abortion has many causes and must not be considered a single disease entity. One of the principal observations in human embryos that fail to cleave normally is the presence of structural abnormalities of chromosomes. In a large series of fetal deaths, the karyotypes of the offspring were compared with the morphology of the conception products.71 More than half of the small or unformed fetuses had chromosomal abnormalities, whereas only 6% of fetuses of normal size with or without malformations had chromosomal aberrations (see Chap. 90). Intrauterine death may occur in association with chromosomal abnormalities that also can be seen in live births. These deaths result from the failure of embryonic development, not the gross anomalies frequently associated in live offspring with the deviant karyotype. There may exist a continuum of anomalies in the offspring into which spontaneous abortion fits, from failure of fetal development through to birth with malformations. Nonchromosomal causes of pregnancy loss
include maternal metabolic disturbances such as endometrial growth factor disturbances or hyperglycemia.72,73