A babys chances for survival increases 3-4% per day between 23 and 24 weeks of gestation and about 2-3% per day between 24 and 26 weeks of gestation. 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. Birth classification
Using gestational age, births can be classified into broad categories:

Using the LMP (last menstrual period) method, a full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week 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 Naegeles rule. 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 (fewer than 28 weeks), very preterm (28 to 32 weeks), moderate to late preterm (32 to 37 weeks). Various jurisdictions may use different classifications.
The actual variability between gestational age as estimated from the beginning of the last menstrual period (without the use of any additional method mentioned in previous section) is substantially larger because of uncertainty which menstrual cycle gave rise to the pregnancy. For example, the menstruation may be scarce enough to give the false appearance that an earlier menstruation gave rise to the pregnancy, potentially giving an estimated gestational age that is approximately one month too large. Also, vaginal bleeding occurs during 15-25% of first trimester pregnancies, and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low. Uses
Gestational age is used for example for:
The events of prenatal development, which usually occur at specific gestational ages. Hence, the gestational 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

Estimation of due date
The mean pregnancy length has been estimated to be 283.4 days of gestational age as timed from the first day of the last menstrual period and 280.6 days when retrospectively estimated by obstetric ultrasound measurement of the fetal biparietal diameter (BPD) in the second trimester.