Gestational age is used for example for: • The events of
prenatal development, which usually occur at specific gestational ages. Hence, the gestational timing of a
fetal toxin exposure,
fetal drug exposure or
vertically transmitted infection can be used to predict the potential consequences to the fetus. •
Estimated date of delivery • Scheduling
prenatal care • Estimation of
fetal viability • Calculating the results of various
prenatal tests, (for example, in the
triple test). • Birth classification into for example preterm, term or postterm. • Classification of infant deaths and stillbirths • Postnatally (after birth) to estimate various risk factors
Estimation of due date The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the
last menstrual period and 280.6 days when retrospectively estimated by
obstetric ultrasound measurement of the
fetal biparietal diameter (BPD) in the second trimester. Other algorithms take into account other variables, such as whether this is the first or subsequent child, the mother's race, age, length of menstrual cycle, and menstrual regularity. In order to have a standard reference point, the normal pregnancy duration is assumed by medical professionals to be 280 days (or 40 weeks) of gestational age. Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. A study of singleton live births determined that childbirth has a
standard deviation of 14 days when gestational age is estimated by first-trimester
ultrasound and 16 days when estimated directly by last menstrual period. According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23
weeks of gestation survive, while 50 to 70 percent of babies born at 24 to 25 weeks, and more than 90 percent born at 26 to 27 weeks, survive. It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. Prognosis depends also on medical protocols on whether to resuscitate and aggressively treat a very premature newborn, or whether to provide only
palliative care, in view of the high risk of severe disability of very preterm babies. , showing
viability and point of 50% chance of survival (
limit of viability) at bottom. Weeks and months numbered by gestation.
Birth classification Using gestational age, births can be classified into broad categories: Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week of gestation is considered to be
preterm. A preterm baby is likely to be
premature and consequently faces increased risk of
morbidity and
mortality. An estimated due date is given by
Naegele's rule. According to the WHO, a preterm birth is defined as "babies born alive before 37 weeks of pregnancy are completed." According to this classification, there are three sub-categories of preterm birth, based on gestational age: extremely preterm (fewer than 28 weeks), very preterm (28 to 32 weeks), moderate to
late preterm (32 to 37 weeks). Babies who were fewer than 28 weeks of gestational age, or weighed fewer than 1000 grams, or fewer than 35 cm in length – even if they showed some sign of life (breathing, heartbeat, voluntary muscle movement) – were classified as "live fetuses" rather than "live births." Only if such newborns survived seven days (168 hours) were they then classified as live births. If, however, they died within that interval, they were classified as stillbirths. If they survived that interval but died within the first 365 days they were classified as infant deaths. More recently, thresholds for "
fetal death" continue to vary widely internationally, sometimes incorporating weight as well as gestational age. The gestational age for statistical recording of fetal deaths ranges from 16 weeks in Norway, to 20 weeks in the US and Australia, 24 weeks in the UK, and 26 weeks in Italy and Spain. The WHO defines the
perinatal period as "The perinatal period commences at 22 completed weeks (154 days) of gestation and ends seven completed days after birth." Perinatal mortality is the death of fetuses or neonates during the perinatal period. A 2013 study found that "While only a small proportion of births occur before 24 completed weeks of gestation (about 1 per 1000), survival is rare and most of them are either fetal deaths or live births followed by a neonatal death." ==Factors affecting pregnancy length==