Early ultrasound gestational age dating

The popularity of using such a definition of gestational age is that menstrual periods are essentially always noticed, while there is usually a lack of a convenient way to discern when fertilization occurred.

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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.

Gestational age is a measure of the age of a pregnancy which is taken from the woman's last menstrual period (LMP), or the corresponding age of the gestation as estimated by a more accurate method if available.

Such methods include adding 14 days to a known duration since fertilization (as is possible in in vitro fertilization), or by obstetric ultrasonography.

After 26 weeks 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.

A preterm baby is likely to be premature and consequently faces increased risk of morbidity and mortality. 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 (less than 28 weeks), very preterm (28 to 32 weeks), moderate to late preterm (32 to 37 weeks).

Various jurisdictions may use different classifications. 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.

According to studies between 20, 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.

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