At what week of gestation is a pregnancy most at risk of miscarriage?

Risk of Miscarriage by Gestational Week

The highest risk of miscarriage occurs during the first 9 weeks of gestation, with the risk being particularly elevated between 0-9 weeks compared to 10 weeks and beyond. 1

Miscarriage Risk by Gestational Age

Early Pregnancy (First Trimester)

• 0-9 weeks: Highest risk period for miscarriage 1
  • Risk is approximately 9.4% at 6 completed weeks of gestation 2
  • Risk decreases to 4.2% at 7 weeks 2
  • Risk further decreases to 1.5% at 8 weeks 2
  • By 9 weeks, risk falls to 0.5% 2

Later First Trimester

• 10+ weeks: Significantly reduced risk compared to earlier weeks
  • Risk is approximately 0.7% at 10 weeks 2
  • After 10 weeks, risk continues to decline 3

Second Trimester

• Miscarriage risk drops substantially after the first trimester
• Weekly miscarriage rates fall below 10 per 1000 woman-weeks by week 14 3
• Rates continue to decrease through week 20 3

Risk Factors That Increase Miscarriage Probability

Maternal Age

• Most significant risk factor for spontaneous abortion 4
• Risk is lower between 25-29 years
• Increases rapidly after age 30
• Substantially elevated at 40-44 years
• Can reach up to 53% at 45 years or older 4

Previous Pregnancy History

• History of previous miscarriage increases risk of recurrent miscarriage:
  • After one miscarriage: adjusted OR of 1.54 (95% CI 1.48-1.60)
  • After two consecutive miscarriages: adjusted OR of 2.21 (95% CI 2.03-2.41)
  • After three consecutive miscarriages: adjusted OR of 3.97 (95% CI 3.29-4.78) 4

Other Risk Factors

• Low body mass index (≤20 kg/m²) 5
• Low serum progesterone concentrations (≤12 ng/ml) 5
• Higher perceived stress levels 5
• Diminished ovarian reserve, particularly in women over 35 4

Clinical Implications

Risk Assessment

• The most critical period for monitoring is during the first 9 weeks of pregnancy 1
• After confirming fetal cardiac activity at 8 weeks, the probability of progressing beyond 20 weeks is very good 2

Special Considerations

• Women with subfertility and previous miscarriages have a higher risk (adjusted RR: 3.1,95% CI: 1.3,7.7) 1
• Women undergoing assisted reproduction have an overall miscarriage rate of 11.8% after confirming viability at 6-8 weeks, with 94.9% of these miscarriages occurring before the 11-14 week scan 6

Monitoring Recommendations

• More intensive monitoring is warranted during the first 9 weeks of pregnancy, especially for women with risk factors
• After 10 weeks with confirmed viability, the risk decreases substantially, allowing for more routine monitoring

Understanding these patterns of miscarriage risk can help guide appropriate clinical monitoring and patient counseling, particularly during the vulnerable early weeks of pregnancy when intervention might be most critical.