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. Other algorithms take into account other variables, such as whether this is the first or subsequent child, the mother's race, age, length of menstrual cycle, and menstrual regularity. 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. 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. 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. Medical fetal viability There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable. According to studies between 2003 and 2005, 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. It is rare for a baby weighing less than 500 g (17.6 ounces) to survive. A baby's 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