Evidence Matters

How Do Maternal Age and Pregnancy History Affect Risk of Miscarriage?

BACKGROUND AND PURPOSE:

Multiple factors have been associated with increased miscarriage risk including
- Parental age | Genetics |Hormonal | Immunologic | Environmental
- Maternal age is the strongest known risk factor
Magnus et al (The BMJ, 2019) sought to
- Estimate the burden of miscarriage in the Norwegian population
- Evaluate whether pregnancy history may also play role in miscarriage

Prospective register-based study (2009-2013)
- Participants: Norwegian women
- Register includes not only birth registry data but also a patient register that provides information on all patient-specialist contact so early losses would also be detected in this system
Primary outcome: risk of miscarriage (losses between 6 and 20 weeks) based on
- Maternal age
- Pregnancy history categorized as
  - No previous pregnancy | Live birth | Stillbirth | Miscarriage | Neonatal death (defined as death within 28 days following delivery)
- Information also obtained on adverse perinatal complications in the previous live birth pregnancy (e.g., preterm birth, preeclampsia)
Statistical analysis: Risk estimates using logistic regression

Maternal age as a risk factor – ‘J shaped pattern’

Overall lowest risk group: 25-29 years (9.8%)
- Absolute lowest risk: Age 27 (9.5%)
High risk groups included oldest and highest in age
- Highest risk group: ≥45 years (53%)
- Youngest mothers: <20 years (15.8%)
- Risk rose rapidly >age 30
No real change when induced abortions were factored in

Pregnancy history as a risk factor

Strong risk of miscarriage if prior miscarriage history is present (compared with the risk of miscarriage in women having their first pregnancy)
- 1 miscarriage: Adjusted odds ratio (aOR) 1.54 (95% CI, 1.48 to 1.60)
- 2 consecutive miscarriages: aOR 2.21 (95% CI, 2.03 to 2.41)
- 3 consecutive miscarriages: aOR 3.97 (95% CI, 3.29 to 4.78)
Risk of miscarriage increased if previous birth ended in
- Preterm birth: aOR 1.22, (95% CI, 1.12 to 1.29)
- Stillbirth: aOR 1.30 (95% CI, 1.11 to 1.53)
- Cesarean section: aOR 1.16 (95% CI 1.12 to 1.21)
Increased risk also seen in the following
- Prior GDM: aOR 1.19 (95% CI, 1.05 to 1.36)
- Women had been SGA themselves: aOR 1.08 (95% CI, 1.04 to 1.13)

Maternal age risk for miscarriage follows a ‘J shaped’ curve
Other pregnancy complications may share underlying mechanisms resulting in associations with miscarriage risk
Relationship to cesarean section is unclear and may reflect that a similar issue giving rise to the need for a cesarean section is the same as that for the subsequent miscarriage (i.e. not causal)

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