First-Trimester Spontaneous Abortion
A 26-year-old woman at 9 weeks gestation presents with cramping and vaginal bleeding and completes a spontaneous miscarriage the same day. This is her first pregnancy loss. She takes no medications, has regular cycles, and has no history of diabetes, thrombophilia, or uterine surgery. Which of the following is the most likely underlying cause of this pregnancy loss?
- A.Fetal chromosomal aneuploidyCorrect
- B.Ascending intrauterine infection
- C.Congenital uterine septum
- D.Undiagnosed maternal hypothyroidism
Explanation
Roughly 50 to 70 percent of first-trimester miscarriages result from fetal chromosomal abnormalities, making aneuploidy by far the single most common cause. Most are sporadic, non-recurrent errors arising during gametogenesis or early cleavage: autosomal trisomies such as trisomy 16 dominate, followed by monosomy X (Turner syndrome) and triploidy. These karyotypes are usually incompatible with continued development, so the embryo arrests and the pregnancy is expelled. The risk rises steeply with maternal age because of meiotic non-disjunction in ageing oocytes, but sporadic aneuploidy also occurs in healthy young women, as here. Because the event is random, a single early loss carries no increased risk for the next pregnancy and does not warrant a workup. The alternative causes cluster elsewhere in gestation: anatomic anomalies and cervical insufficiency typically cause second-trimester or recurrent losses, infection is a more important contributor later in pregnancy, and endocrine or autoimmune disorders are sought only after recurrent loss. Investigation for antiphospholipid syndrome, uterine anatomy, and parental karyotype is reserved for two or more consecutive losses. Counselling should therefore be reassuring and emphasise that nothing she did caused this.
Why each option
- A.
- Correct. Chromosomal abnormalities, mostly autosomal trisomies and monosomy X, cause 50 to 70 percent of first-trimester losses and are typically sporadic.
- B.
- Infection contributes to mid-trimester and later loss and rarely explains an isolated sporadic first-trimester miscarriage in a well woman.
- C.
- Uterine anomalies such as a septate uterus classically cause second-trimester and recurrent losses rather than a single early miscarriage.
- D.
- Overt thyroid disease can increase miscarriage risk but accounts for a very small minority of early losses and would usually be symptomatic.
Reference: Williams Obstetrics, 26th ed., 2022, Abortion
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