Primipara-a woman who has been delivered only once of a fetus or fetuses born alive or dead with an estimated length of gestation of 20 or more weeks. In the past, a 500-g birthweight threshold was used to define parity. his threshold is now controversial because many states still use this weight to diferentiate a stillborn fetus from an abortus (Chap. 1, p. 3). However, the survival of neonates with birthweights < 500 g is no longer uncommon.
5. Multpara-a woman who has completed two or more pregnancies to 20 weeks' gestation or more. Parity is determined by the number of pregnancies reaching 20 weeks. It is not increased to a higher number if multiples are delivered in a given pregnancy. Moreover, stillbirth does not lower this number. In some locales, the obstetrical history is summarized by a series of digits connected by dashes. hese refer to the number of term infants, preterm infants, abortuses younger than 20 weeks, and children currently alive. For example, a woman who is para 2-1-0-3 has had two term deliveries, one preterm delivery, no abortuses, and has three living children. Because these are nonconventional, it is helpful to speciy the outcome of any pregnancy that did not end normally.
The normal duration of pregnancy calculated from the irst day of the last normal menstrual period is very close to 280 days or 40 weeks. In a study of 427,581 singleton pregnancies from the Swedish Birth Registry, Bergsj0 and coworkers (1990) found that the mean pregnancy duration was 281 days with a standard deviation of 13 days. However, menstrual cycle length varies among women and renders many of these calculations inaccurate. This, combined with the frequent use of irst-trimester sonography, has changed the method of determining an accurate gestational age (Duryea, 2015).
he American College of Obstetricians and Gynecologists (2017e), the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine have concluded that irst-trimester ultrasound is the most accurate method to establish or reairm gestational age. For pregnancies conceived by assisted reproductive technology, embryo age or transfer date is used to assign gestational age. If available, the gestational ages calculated from the last menstrual period and from irst-trimester ultrasound are compared, and this estimated date of delivery is recorded. his is discussed in further detail in Chapter 7 (p. 124) and in Table 10-1 (p. 183).
A quick estimate of a pregnancy due date based on menstrual data can be made as follows: add 7 days to the irst day of the last period and subtract 3 months. For example, if the irst day of the last menses was October 5, the due date is 10-05 minus 3 (months) plus 7 (days) = 7-12, or July 12 of the following year. his calculation is the Naegele rule (American College of Obstetricians and Gynecologists, 2017 e).
It has become customary to divide pregnancy into three equal epochs or trimesters of approximately 3 calendar months. Historically, the irst trimester extends through completion of 14 weeks, the second through 28 weeks, and the third includes the 29th through 42nd weeks of pregnancy. Thus, there are three periods of 14 weeks each. Certain major obstetrical problems tend to cluster in each of these time periods. For example, most spontaneous abortions take place during the irst trimester, whereas most women with hypertensive disorders due to preg nancy are diagnosed during the third trimester.