Gestational age

According to American College of Obstetricians and Gynecologists, the main methods to calculate gestational age are:[2] 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.

[5] Once the estimated due date (EDD) is established, it should rarely be changed, as the determination of gestational age is most accurate earlier in the pregnancy.

Also, vaginal bleeding occurs during 15–25% of first trimester pregnancies,[10] and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.

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.

[11] 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 days to the first day of a woman's last menstrual period (LMP) or corresponding date as estimated from other means.

There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable.

[15] 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.

More recently, thresholds for "fetal death" continue to vary widely internationally, sometimes incorporating weight as well as gestational age.

For example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation.

Also, preexisting maternal medical conditions with genetic component, e.g., diabetes mellitus type 1, systemic lupus erythematosus, anaemia.

[30] Family-based studies showed that gestational age at birth is partially (25–40%) determined by genetic factors.

Timeline of pregnancy by gestational age
Distribution of gestational age at childbirth among singleton live births, given both when gestational age is estimated by first trimester ultrasound and directly by last menstrual period [ 11 ]
Stages in prenatal development , showing viability and point of 50% chance of survival ( limit of viability ) at bottom. Weeks and months numbered by gestation.