Operational Definition of Parity
Parity is defined as the number of pregnancies reaching viability (typically ≥24 weeks gestation) that have been delivered, regardless of the number of fetuses or whether the infant was born alive or stillborn, with multiple gestations (twins, triplets, etc.) counted as a single parous event.
Current State of Definition
Despite widespread use in obstetrics, there is no universally standardized definition of parity, and significant confusion exists among healthcare providers 1, 2:
- A 2021 survey in Wales found that only 5% of doctors but 49% of midwives failed to include stillbirths after 24 weeks in their definition of parity 1
- When asked about twin deliveries, 84% of surveyed clinicians correctly described a previous twin delivery as Para 1 (counting it as a single parous event), but 21% in an earlier 2007 survey incorrectly counted twins as Para 2 1, 3
- A literature review identified 23 different reference sources defining parity, with variations describing it as the number of conceptions, pregnancies, births, or babies 1
Key Components of the Operational Definition
Gestational Age Threshold
- Viability is typically defined as ≥24 weeks of completed gestation 1
- The 20-24 week period represents a "gray zone" where births and abortions can be confused, leading to inconsistent documentation 2
Multiple Gestations
- Multiple births (twins, triplets, etc.) should be counted as a single parous event 1, 4
- Of 12 sources that addressed multiple pregnancy, 8 defined multiple births as a single parous experience 1
- This is clinically relevant as parity affects risk stratification in twin pregnancies 4
Stillbirths
- Stillbirths after 24 weeks should be included in parity count 1
- This remains an area of confusion, particularly among midwives 1
Clinical Risk Stratification by Parity
For practical clinical application, women can be classified into three risk groups based on parity 5:
- Nulliparity (Para 0): Higher risk for obstetric complications, neonatal morbidity, and perinatal mortality 5
- Low multiparity (Para 1-3): Lowest risk group, serving as the reference standard 5
- Grand multiparity (Para ≥4): Increased risk compared to low multiparity, though the definition should start at parity 4 or the 5th baby 5
Common Pitfalls
- Inconsistent documentation: The lack of standardization leads to confusion in medical records and communication between providers 1, 2, 3
- Multiple gestation confusion: Counting each fetus rather than each pregnancy leads to inaccurate risk assessment 3
- Viability threshold variability: Different institutions may use different gestational age cutoffs 2