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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 days when gestational age is estimated by first trimester ultrasound, and 16 days when estimated directly by last menstrual period.
More recently, thresholds for "fetal death" continue to vary widely internationally, sometimes incorporating weight as well as gestational age.
Babies who were less than 28 weeks of gestational age, or weighed less than 1000 grams, or less 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.
A 2013 study found that "While only a small proportion of births occur before 24 completed weeks of gestation (about ), survival is rare and most of them are either fetal deaths or live births followed by a neonatal death." Gestational age (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.