What is a threatened abortion?

What is a Threatened Abortion?

Threatened abortion is vaginal bleeding occurring before 20 weeks of gestation with a closed cervix and a viable intrauterine pregnancy, representing a clinical scenario where the pregnancy may continue to term or progress to pregnancy loss. 1, 2

Clinical Definition and Diagnostic Criteria

Threatened abortion is characterized by the following key features:

• Vaginal bleeding during the first half of pregnancy (before 20 weeks gestation), which can range from light spotting to heavier bleeding 1, 2
• Closed cervix on speculum examination, distinguishing it from inevitable abortion where cervical dilation has occurred 1
• Viable fetus with documented cardiac activity on ultrasound, confirming ongoing pregnancy 3, 2
• Uterine cramping may or may not be present alongside the bleeding 2

Epidemiology and Natural History

Threatened abortion is the most common complication in early pregnancy:

• Occurs in approximately 15% of all clinically recognized pregnancies 2
• Less than 30% of women with threatened abortion will progress to spontaneous abortion, meaning the majority (70-75%) of pregnancies continue to term 2, 4
• Fetomaternal hemorrhage occurs in 48% of threatened abortion cases, with an overall rate of 32% in patients undergoing spontaneous abortion 3

Ultrasound Findings

Transvaginal ultrasonography is the diagnostic method of choice and may reveal:

• Subchorionic hematoma (blood collection between the membranes and uterine wall), found in approximately 42.9% of cases 4, 5
• Viable intrauterine pregnancy with documented fetal cardiac activity 3
• Hematomas less than 35 ml typically resolve with pregnancy continuing to term, while those greater than 50 ml are associated with higher rates of abortion or preterm delivery 5

Distinction from Other First-Trimester Conditions

The 2024 Society of Radiologists in Ultrasound consensus guidelines clarify important terminology distinctions 6:

• Threatened abortion is NOT classified as "Early Pregnancy Loss (EPL)" because the pregnancy remains viable with cardiac activity present
• It differs from "Concerning for EPL" which applies when there is an embryo <7 mm without cardiac activity or other poor prognostic features
• It is distinct from "Pregnancy of Unknown Location (PUL)" where no definite intrauterine or ectopic pregnancy is visualized 6

Associated Pregnancy Complications

Pregnancies complicated by threatened abortion constitute a high-risk group with increased rates of:

• Spontaneous preterm delivery (25.2% vs 9.4% in controls) 7
• Premature rupture of membranes (PROM) (10.2% vs 4.8% in controls) 7
• Placental abruption (4% vs 1.4% in controls) 7
• Intrauterine growth restriction (IUGR) (13.2% of continuing pregnancies) 4
• Lower neonatal birth weight in term pregnancies (3046g vs 3317g in controls) 7
• The risk increases with multiple bleeding episodes and greater bleeding volume 7

Critical Management Consideration: Rh Status

All Rh-negative women with threatened abortion require careful evaluation for anti-D immunoglobulin administration 3:

• The American College of Obstetricians and Gynecologists states there is no evidence-based recommendation for anti-D immunoglobulin administration in threatened abortion with a viable fetus, and many physicians do not treat when there is a live embryo or fetus 3
• However, if pregnancy loss occurs, 50 μg of anti-D immunoglobulin must be administered to prevent Rh-D alloimmunization 3

Common Pitfall to Avoid

Do not confuse threatened abortion with other forms of early pregnancy complications 6, 1:

• In myeloproliferative neoplasms literature, "threatened abortion" is sometimes used loosely to describe any signs/symptoms of potential pregnancy loss (pain or bleeding), but the proper medical definition requires a closed cervix and viable pregnancy 6
• Always confirm viability with ultrasound before using this terminology, as the presence or absence of cardiac activity fundamentally changes the diagnosis and management 6