Operational Definition of Parity in Obstetrics
Parity is defined as the number of pregnancies reaching viable gestational age (typically ≥20-24 weeks) that a woman has delivered, regardless of the number of fetuses delivered or whether the infant was born alive or stillborn, with multiple gestations (twins, triplets) counted as a single parous event.
Standard Definition Components
The operational definition of parity includes the following key elements:
Parity counts deliveries, not babies: A woman who delivers twins is considered para 1, not para 2, as the delivery event itself is what defines parity 1, 2.
Gestational age threshold: Parity includes pregnancies delivered at or beyond viability, typically defined as ≥20-24 weeks of gestation, though this cutoff varies by institution and country 3.
Stillbirths are included: Deliveries of stillborn infants after the viability threshold count toward parity, though significant confusion exists among clinicians—49% of midwives surveyed incorrectly excluded stillbirths after 24 weeks from their parity definition 1.
Clinical Application and Common Pitfalls
Major inconsistencies exist in clinical practice: Only 21% of obstetricians and midwives correctly described twin delivery as a single parous experience in one survey, demonstrating widespread misapplication of the term 2.
Key Areas of Confusion:
Multiple gestations: The most common error is counting each baby rather than each delivery event—84% of surveyed clinicians incorrectly described a previous twin delivery as para 2 1.
The "gray zone" of 20-24 weeks: Births and abortions can be confused in this gestational age range, as different institutions use different viability thresholds 3.
Lack of standardization: A literature review identified 23 different referenced definitions of parity, with variability in whether it counted conceptions, pregnancies, births, or babies 1.
Clinical Risk Stratification by Parity
Understanding parity definitions is clinically important for risk assessment:
Nulliparity (para 0): Associated with higher risk of obstetric complications, neonatal morbidity, and perinatal mortality 4.
Low multiparity (para 1-3): Represents the lowest risk group for maternal and neonatal complications 4.
Grand multiparity (para ≥4): Traditionally defined as parity of 5 or more, but evidence supports defining it as parity ≥4, as this group shows increased risk compared to low multiparity 4.