Clinical Definition of Parity
Parity is defined as the number of times a woman has given birth to a fetus at or beyond the threshold of viability (typically ≥24 weeks gestation or ≥500g), regardless of whether the infant was born alive or stillborn, with multiple gestations (twins, triplets) counted as a single parous event.
The Definition Problem
There is significant confusion and lack of standardization in how parity is defined and applied clinically:
A 2021 survey of obstetricians and midwives in Wales revealed alarming inconsistencies: 49% of midwives and 5% of doctors incorrectly excluded stillbirths after 24 weeks from their definition of parity, while only 84% correctly described a previous twin delivery as Para 1 (not Para 2) 1.
Literature review identified 23 different definitions of parity across professional sources, with variability in whether it refers to number of conceptions, pregnancies, births, or babies 1.
Only 12 sources addressed multiple pregnancies, and of these, 8 correctly defined multiple births as a single parous event 1.
Standard Clinical Application
What Counts Toward Parity:
- Any delivery at ≥24 weeks gestation (or ≥20-24 weeks depending on local viability definitions) 2
- Stillbirths after the viability threshold are included 1
- Multiple gestations count as ONE parous event, not one per fetus 1, 3
What Does NOT Count Toward Parity:
- Miscarriages or abortions before viability threshold (typically <20-24 weeks) 2
- Ectopic pregnancies
- Molar pregnancies
Clinical Significance by Parity Level
Understanding parity matters for risk stratification:
Nulliparity (Para 0): Higher risk for preterm birth, low birth weight, small for gestational age, and obstetric complications 4, 5
Low multiparity (Para 1-3): Lowest risk group for adverse outcomes 4
Grand multiparity (Para ≥4): Increased risk returns, with higher obstetric complications, neonatal morbidity, and perinatal mortality 4
Common Pitfalls to Avoid
- Counting twins as Para 2: This is incorrect; a twin delivery is Para 1 1, 3
- Excluding stillbirths: Stillbirths after viability threshold must be included in parity 1
- Confusion in the 20-24 week gray zone: Terminology often fails to account for local definitions of viability, leading to inconsistent documentation 2
- Only 21% of surveyed clinicians correctly applied parity terminology in multiple pregnancies, demonstrating widespread misunderstanding 3
Documentation Recommendation
The lack of standardization creates communication errors and documentation inconsistencies 1, 3, 2. When documenting parity, be explicit about:
- Gestational age at each delivery
- Whether multiple gestations occurred
- Outcome of each pregnancy (live birth vs stillbirth)