If the gestational age as calculated from an early ultrasound is contradictory to the one calculated directly from the last menstrual period, it is still the one from the early ultrasound that is used for the rest of the pregnancy. In case of in vitro fertilization, calculating days since oocyte retrieval or co-incubation and adding 14 days.Gestational age can also be estimated by calculating days from ovulation if it was estimated from related signs or ovulation tests, and adding 14 days by convention.A more complete listing of methods is given in following table:
As a general rule, the official gestational age should be based on the actual beginning of the last menstrual period, unless any of the above methods gives an estimated date that differs more than the variability for the method, in which case the difference cannot probably be explained by that variability alone.
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." Postnatal use
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. Factors affecting pregnancy length
Childs gestational age at birth (pregnancy length) is associated with various likely causal maternal non-genetic factors: stress during pregnancy, age, parity, smoking, infection and inflammation, BMI. Also, preexisting maternal medical conditions with genetic component, e.g., diabetes mellitus type 1, systemic lupus erythematosus, anaemia. Parental ancestral background (race) also plays a role in pregnancy duration. Gestational age at birth is on average shortened by various pregnancy aspects: twin pregnancy, prelabor rupture of (fetal) membranes, pre-eclampsia, eclampsia, intrauterine growth restriction. The ratio between fetal growth rate and uterine size (reflecting uterine distension) is suspected to partially determine the pregnancy length. Heritability of pregnancy length
Family-based studies showed that gestational age at birth is partially (from 25% to 40%) determined by genetic factors. See also
Pregnancy
Maternity
Prenatal development
Gestation periods in mammals
Abortion law
Reproductive rights
Fetal rights


== References ==