What are the risk factors and when does a silent (missed) miscarriage typically occur?

Silent Miscarriage: Timing and Risk Factors

Silent miscarriage (missed miscarriage) typically occurs in the first trimester, often between 6-10 weeks, when an embryo has died but has not been expelled from the uterus, and several risk factors including advanced maternal age, previous miscarriages, and diminished ovarian reserve increase the likelihood of occurrence.

Definition and Timing

• Silent miscarriage, also termed "missed miscarriage," occurs when the embryo has died but has not been expelled from the uterus, corresponding to "diagnostic of early pregnancy loss (EPL)" in current terminology 1
• Most silent miscarriages occur during the first trimester (before 14 weeks gestation), with the majority detected between 6-10 weeks during routine ultrasound examinations 1, 2
• Silent miscarriages are often discovered incidentally during routine prenatal care when ultrasound reveals no fetal heartbeat despite a previously confirmed pregnancy 2

Ultrasound Diagnostic Criteria

• Diagnostic criteria for silent miscarriage include: embryonic crown-rump length (CRL) ≥7 mm with no cardiac activity, mean sac diameter (MSD) ≥25 mm with no embryo, or absence of embryo ≥14 days after visualization of gestational sac 1
• Concerning findings that may suggest impending silent miscarriage include: embryonic CRL <7 mm with no cardiac activity, MSD 16-24 mm with no embryo, or absence of embryo 7-13 days after visualization of gestational sac 1
• Poor prognostic indicators include a calcified yolk sac and an enlarged amniotic cavity relative to the crown-rump length (expanded amnion sign) 1

Risk Factors

Maternal Factors

• Advanced maternal age (>35 years) significantly increases risk of silent miscarriage 3
• Very young maternal age (<20 years) is also associated with increased risk 3
• Women with diminished ovarian reserve, characterized by low anti-Müllerian hormone (AMH) levels, have approximately 35% increased relative risk of miscarriage 4
• Previous miscarriages, especially recurrent miscarriages, increase the risk of subsequent pregnancy loss 3
• Extremes of body mass index (very low or very high) are associated with increased risk 3
• Black ethnicity has been identified as a risk factor for miscarriage 3

Lifestyle and Environmental Factors

• Smoking increases the risk of miscarriage, including silent miscarriage 3
• Alcohol consumption is associated with higher miscarriage rates 3
• High levels of psychological stress contribute to increased risk 3
• Working night shifts has been linked to higher miscarriage rates 3
• Environmental exposures including air pollution and pesticides increase risk 3

Paternal Factors

• Advanced paternal age (>40 years) is associated with increased risk of miscarriage 3

Clinical Implications and Management

• Silent miscarriage can be managed expectantly (waiting for natural expulsion), medically (using medications like misoprostol), or surgically (suction aspiration or dilation and curettage) 5, 6
• Medical management with misoprostol has shown effectiveness with complete miscarriage rates similar to surgical management in many cases 7, 6
• Surgical methods generally rank highest for managing a miscarriage, followed by medical methods, which in turn rank above expectant management 5
• Recurrent miscarriage serves as a sentinel marker for various obstetric risks in future pregnancies, including preterm birth, fetal growth restriction, placental abruption, and stillbirth 3
• Women with history of miscarriage have increased long-term health risks including cardiovascular disease and venous thromboembolism 3

Psychological Impact

• Miscarriage, including silent miscarriage, can lead to significant psychological consequences including anxiety, depression, post-traumatic stress disorder, and in severe cases, increased risk of suicide 3
• Psychological effects may last up to 12 months after diagnosis 2

Important Considerations

• Approximately 15% of clinically recognized pregnancies end in miscarriage, with silent miscarriage being a common subtype 3, 5
• The global burden of miscarriage is substantial, with an estimated 23 million miscarriages occurring worldwide annually 3
• Complete miscarriage should not be diagnosed with transvaginal ultrasound alone without serial biochemical confirmation (unless an intrauterine gestation sac has previously been visualized) 2