Abortion: Definitions, Classifications, and Etiology

Abortion is defined as the spontaneous or induced termination of pregnancy before fetal viability. Specifically, it is the process of partial or complete separation of the product of conception from the uterine wall with or without partial or complete expulsion from the uterine cavity before the age of viability. The World Health Organization define abortion as pregnancy termination before 20 weeks’ gestation or with a fetus born weighing < 500 g.

Classification of Abortion

There are several types of abortion classified based on clinical symptoms and cervical dilation:

• Threatened abortion
• Inevitable abortion
• Incomplete abortion
• Complete abortion
• Missed abortion

Furthermore, recurrent abortion is defined as 2-3 consecutive losses. Induced abortion is defined as termination of pregnancy before viability, typically measured through abortion ratio and rate.

Spontaneous Abortion

Spontaneous abortion is the most common complication of pregnancy and is defined as the passing of a pregnancy prior to completion of the 20th gestational week. It implies delivery of all or any part of the products of conception, with or without a fetus weighing less than 500g. Statistics show that 80% of spontaneous abortions occur prior to 12 weeks' gestation. The incidence of abortion is influenced by the age of the mother and by a number of pregnancy-related factors, including a history of a previous full-term normal pregnancy, the number of previous spontaneous abortions, a previous stillbirth, and a previous infant born with malformations or known genetic defects.

Etiology and Chromosomal Factors

The primary causes of spontaneous abortion include chromosomal abnormalities and anatomical defects. An abnormal karyotype is present in approximately 50% of spontaneous abortions occurring during the first trimester. The incidence of these abnormalities decreases to 20–30% in second-trimester losses and to 5–10% in third trimester losses.

Embryonic Development

Approximately half of miscarriages are anembryonic, that is, with no identifiable embryonic elements. A blighted ovum represents a failed development of the embryo so that only a gestational sac, with or without a yolk sac, is present. The other 50 percent are embryonic miscarriages, which commonly display a developmental abnormality of the zygote, embryo, fetus, or at times, the placenta.

Aneuploid vs Euploid Abortion

• Aneuploid Abortion: This occurs at earlier gestational ages; 75 percent of aneuploid abortions occurred by 8 weeks.
• Euploid Abortion: Chromosomally normal fetuses abort later than those that are aneuploid. Specifically, the rate of euploid abortions peaks at approximately 13 weeks.

Maternal Factors

Various maternal conditions and infections are implicated in spontaneous abortion:

• Infections: Organisms such as Treponema pallidum, Chlamydia trachomatis, Neisseria gonorrhoeae, Streptococcus agalactiae, herpes simplex virus, cytomegalovirus, and Listeria monocytogenes.
• Medical Conditions: Endocrine disorders such as hyperthyroidism and poorly controlled diabetes mellitus; cardiovascular disorders, such as hypertensive or renal disease; and connective tissue disease, such as systemic lupus erythematosus.
• Uterine Defects: Congenital anomalies that distort or reduce the size of the uterus.