Gestational age

In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period, 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, or by obstetric ultrasonography. The popularity of using this measure of pregnancy is largely due to convenience: menstruation is usually noticed, while there is generally no convenient way to discern when fertilization or implantation occurred.
Gestational age is contrasted with fertilization age, which takes the date of fertilization as the start date of gestation. There are different approaches to defining the start of a pregnancy. This definition is unusual in that it describes women as becoming "pregnant" about two weeks before they even had intercourse. The definition of pregnancy and the calculation of gestational age are also relevant in the context of the abortion debate and the philosophical debate over the beginning of human personhood.

Methods

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

Directly calculating the days since the beginning of the last menstrual period
Early obstetric ultrasound, comparing the size of an embryo or fetus to that of a reference group of pregnancies of known gestational age and using the mean gestational age of other embryos or fetuses of the same size. 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:

Method of estimating gestational age	Variability
Days from oocyte retrieval or co-incubation in in vitro fertilisation + 14 days	±1 day
Days from estimated ovulation in ovulation induction + 14 days	±3 days
Days from artificial insemination + 14 days	±3 days
Days from known single sexual intercourse + 14 days	±3 days
Days from estimated ovulation by basal body temperature record + 14 days	±4 days
First-trimester physical examination	±2 weeks
Second-trimester physical examination	±4 weeks
Third-trimester physical examination	±6 weeks
First-trimester obstetric ultrasonography	±8% of the estimate
Second-trimester obstetric ultrasonography	±8% of the estimate
Third-trimester obstetric ultrasonography	±8% of the estimate

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. For example, if there is a gestational age based on the beginning of the last menstrual period of 9.0 weeks, and a first-trimester obstetric ultrasonography gives an estimated gestational age of 10.0 weeks, the difference of 1.0 weeks between the tests is larger than the 2 SD variability of the ultrasonography estimate, indicating that the gestational age estimated by ultrasonography should be used as the official gestational age.
Once the estimated due date is established, it should rarely be changed, as the determination of gestational age is most accurate earlier in the pregnancy.
Assessment of gestational age can be made based on selected head and trunk parameters. Following are diagrams for estimating gestational age from obstetric ultrasound, by various target parameters:

Comparison to fertilization age

The fertilization or conceptional age is the time from the fertilization. It usually occurs within a day of ovulation, which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation.
There is also considerable variability in this interval, with a 95% prediction interval of the ovulation of 9 to 20 days after menstruation even for an average woman who has a mean LMP-to-ovulation time of 14.6. In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days. The actual variability between gestational age as estimated from the beginning of the last menstrual period 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.

Event	Gestational age	Fertilization age	Implantation age
Menstrual period begins	Day 1 of pregnancy	Not pregnant	Not pregnant
Has sex and ovulates	2 weeks pregnant	Not pregnant	Not pregnant
Fertilization; cleavage stage begins	Day 15	Day 1	Not pregnant
Implantation of blastocyst begins	Day 20	Day 6	Day 0
Implantation finished	Day 26	Day 12	Day 6
Embryo stage begins; first missed period	4 weeks	Day 15	Day 9
Primitive heart function can be detected	5 weeks, 5 days	Day 26	Day 20
Fetal stage begins	10 weeks, 1 day	8 weeks, 1 day	7 weeks, 2 days
First trimester ends	13 weeks	11 weeks	10 weeks
Second trimester ends	26 weeks	24 weeks	23 weeks
Childbirth	39–40 weeks	37–38 weeks	36–37 weeks

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,.
Birth classification into for example preterm, term or postterm.
Classification of infant deaths and stillbirths
Postnatally 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 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 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 determined 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 by adding a year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period 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 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.

Completed weeks of gestation at birth	21 and less	22	23	24	25	26	27	30	34
Chance of survival	<1%	0–10%	10–35%	40–70%	50–80%	80–90%	>90%	>95%	>98%

Birth classification

Using gestational age, births can be classified into broad categories:

Gestational Age in Weeks	Classification
< 37 0/7	Preterm
34 0/7 - 36 6/7	Late preterm
37 0/7 - 38 6/7	Early Term
39 0/7 - 40 6/7	Full Term
41 0/7 - 41 6/7	Late Term
> 42 0/7	Postterm

Using the LMP method, a full-term human pregnancy is considered to be 40 weeks, 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 Naegele's 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, very preterm, moderate to late preterm. Various jurisdictions may use different classifications.