At what point during gestation can spontaneous abortion (miscarriage) occur in a female patient of childbearing age with no specified medical history or underlying conditions?

Spontaneous Abortion Timing During Gestation

Spontaneous abortion is defined as pregnancy loss occurring before 20 weeks' gestation, with some definitions extending to 28 weeks depending on the classification system used. 1, 2

Gestational Age Definition and Boundaries

• The standard medical definition establishes spontaneous abortion as occurring before 20 weeks' gestation, which is the most widely accepted threshold in clinical practice 1, 3

• Some classification systems extend this definition to 28 weeks' gestation, though the 20-week cutoff remains the predominant standard 2

• Fetal viability considerations typically begin around 20-24 weeks, which is why this gestational age serves as the dividing line between spontaneous abortion and other pregnancy loss categories 1

Critical Physiologic Timeline

• Fetal red blood cells display RhD antigens from as early as 6 weeks of gestation, making maternal sensitization physiologically possible even in very early pregnancy—this is clinically relevant for Rh-negative women requiring RhIg prophylaxis 4, 5

• Chromosomal abnormalities account for 50-60% of spontaneous abortions, with most occurring in the first trimester 5, 1

• The risk of maternal mortality from spontaneous abortion increases significantly with advancing gestational age: women at gestational age over 12 weeks have an 8-fold increased risk of death compared to earlier losses (RR 8.0,95% CI 4.2-11.9) 3

Clinical Risk Stratification by Trimester

First Trimester (up to 12-13 weeks):

• Lower maternal morbidity and mortality risk 6, 3
• RhIg dose of 50 μg is adequate for pregnancy losses in this period 4, 7
• Most spontaneous abortions occur during this timeframe due to chromosomal abnormalities 5, 1

Second Trimester (14-20 weeks):

• Significantly increased risk of maternal complications including hemorrhage, infection, and disseminated intravascular coagulation (DIC) 6, 3
• Major abortion complication rates increase from 0.8% at 11-12 weeks to 2.2% at 17-20 weeks 6
• Maternal mortality risk increases from 1.1 deaths per 100,000 at 11-12 weeks to 6.9 deaths per 100,000 at 16-20 weeks 6
• Standard 300 μg RhIg dose required for Rh-negative women 4

Common Clinical Pitfall

Do not assume that early gestational age eliminates the risk of maternal sensitization or complications—fetal RBCs with D-antigen are present from 6 weeks onward, and even first-trimester losses can result in fetomaternal hemorrhage in 32% of cases 4, 5