<unk>  is used for example for: *The events of prenatal development, which usually occur at specific<unk> s. Hence, the<unk>  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 Timeline of pregnancy by<unk> . Estimation of due date Distribution of<unk>  at childbirth among singleton live births, given both when<unk>  is estimated by first trimester ultrasound and directly by last menstrual period The mean pregnancy length has been estimated to be 283.4<unk>  of<unk>  as timed from the first day of the last menstrual period and 280.6<unk>  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<unk>  (or 40<unk>  of<unk> . Furthermore, actual childbirth has only a certain probability of occurring within the limits of the estimated due date. A study of singleton live births came to the result that childbirth has a standard deviation of 14<unk>  when<unk>  is estimated by first-trimester ultrasound and 16<unk>  when estimated directly by last menstrual period. The most common system used among healthcare professionals is Naegele's rule, which estimates the expected date of delivery (EDD) by adding a year, subtracting three months, and adding seven<unk>  to the first day of a woman's last menstrual period (LMP) or corresponding date as estimated from other means. Medical fetal viability There is no sharp limit of development,<unk> , or weight at which a human fetus automatically becomes viable. According to studies between 2003 and 2005, 20 to 35 percent of babies born at 23<unk>  of gestation survive, while 50 to 70 percent of babies born at 24 to 25<unk> , and more than 90 percent born at 26 to 27<unk> , survive. It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. A baby's chances for survival increases 3-4% per day between 23 and 24<unk>  of gestation and about 2-3% per day between 24 and 26<unk>  of gestation. After 26<unk>  the rate of survival increases at a much slower rate because survival is high already. 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. Stages in prenatal development, showing viability and point of 50% chance of survival (limit of viability) at bottom.<unk>  and months numbered by gestation, Completed<unk>  of gestation at birth 21 and less 22 23 24 25 26 27 30 34 Chance of survival 90% >95% >98% Birth classification Using<unk> , births can be classified into broad categories:<unk>  in<unk>  Classification 42 0/7 Postterm Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40<unk>  (280<unk> , though pregnancy lengths between 38 and 42<unk>  are considered normal. A fetus born prior to the 37th<unk> 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<unk>  of pregnancy are completed." According to this classification, there are three sub-categories of preterm birth, based on<unk> : extremely preterm (fewer than 28<unk> , very preterm (28 to 32<unk> , moderate to late preterm (32 to 37<unk> . Various jurisdictions may use different classifications. In classifying perinatal deaths, stillbirths and infant deaths For most of the 20th Century, official definitions of a live birth and infant death in the Soviet Union and Russia differed from common international standards, such as those established by the World Health Organization in the latter part of the century. Babies who were fewer than 28<unk>  of<unk> , 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<unk>  (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<unk>  they were classified as infant deaths. More recently, thresholds for "fetal death" continue to vary widely internationally, sometimes incorporating weight as well as<unk> . The<unk>  for statistical recording of fetal deaths ranges from 16<unk>  in Norway, to 20<unk>  in the US and Australia, 24<unk>  in the UK, and 26<unk>  in Italy and Spain. The WHO defines the perinatal period as "The perinatal period commences at 22 completed<unk>  (154<unk>  of gestation and ends seven completed<unk>  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<unk>  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." Postnatal use<unk>  (as well as fertilization age) is sometimes used postnatally (after birth) to estimate various risk factors. For example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation.
Gestational age
Metabolic age
Prenatal nutrition
Birth weight
Infant formula
Birthmark
Childbirth positions
Sex after pregnancy