A commonly used clinical definition of parity is the number of births (both live born infants and stillbirths) of at least 20 weeks of gestation that a woman has experienced. Nulliparas have experienced no such births, primiparas have experienced one such birth, and multiparas have experienced more than one such birth. (Pregnancy losses under 20 weeks of gestation are considered abortions, which may be spontaneous or induced. A woman's gravidity is the sum of her parity and number of abortions).
A 1934 study suggested that multiparity increased the risk of pregnancy complications, and first introduced the term "grand multipara." The author concluded that grand multiparity was dangerous because, in his study, maternal mortality increased steadily from the 5th to the 10th pregnancy . On the basis of this finding, and results from subsequent studies, the clinically accepted definition of grand multiparity became parity equal to or greater than five. In the United States in 2011, nearly 190,000 women gave birth to a child that was at least their fifth birth .
VARIABLES AFFECTING THE RELATIONSHIP BETWEEN PARITY AND PREGNANCY OUTCOME
The relationship between obstetric complications and parity has been studied extensively, with inconsistent findings. In additional to the usual limitations of observational data, there are several reasons for the discordancy:
Variability in the definition of parity — There are discrepancies in the way clinicians define parity. A study that evaluated the definition of the term parity among obstetricians and midwives in London noted that 13 percent defined parity by the number of pregnancies irrespective of outcome, 62 percent defined parity as pregnancies of ≥24 weeks of gestation irrespective of outcome, and 25 percent defined parity as pregnancies of ≥24 weeks ending in live birth . Only 21 percent of those surveyed described twin pregnancies as a single parous experience.
In addition, there is a discrepancy between the clinical definition of parity and the definition used by the United States National Center for Health Statistics (NCHS), an organization within the Centers for Disease Control. The NCHS defines parity as the number of prior live births, excluding stillbirths and fetal deaths , while most clinical investigators in the US have defined parity as the number of births (live births and stillbirths) >20 weeks of gestation. Other gestational age cut-offs to define a parous event (eg, birth after 28 weeks of gestation) are used in other parts of the world.