In obstetrics, gestational age is a measure of the age of a pregnancy taken from the beginning of the woman's last menstrual period (LMP), 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 (as is possible in in vitro 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
- Directly calculating the days since the beginning of the last menstrual period
- Early obstetric ultrasound, comparing the size of an human embryo or fetus to that of a reference group of pregnancies of known gestational age (such as calculated from last menstrual periods) 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 , and adding 14 days by convention.
A more complete listing of methods is given in following table: A Simple Solution to Dating Discrepancies: The Rule of Eights
| ±1 day |
| ±3 days |
| ±3 days |
| ±3 days |
| ±4 days |
| ±2 weeks |
| ±4 weeks |
| ±6 weeks |
| ±8% of the estimate |
| ±8% of the estimate |
| ±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 (with a 2 SD variability of ±8% of the estimate, thereby giving a variability of ±0.8 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 (EDD) 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.
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.Derived from a standard deviation in this interval of 2.6, as given in: 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 (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 Pregnancy, and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.
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.
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 birth and consequently faces increased risk of morbidity and death. 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 (fewer than 28 weeks), very preterm (28 to 32 weeks), moderate to late preterm (32 to 37 weeks). Various jurisdictions may use different classifications.
More recently, thresholds for "fetal death" continue to vary widely internationally, sometimes incorporating weight as well as gestational age. The gestational age for statistical recording of fetal deaths ranges from 16 weeks in Norway, to 20 weeks in the US and Australia, 24 weeks in the UK, and 26 weeks in Italy and Spain.
The WHO defines the perinatal period as "The perinatal period commences at 22 completed weeks (154 days) of gestation and ends seven completed days after birth."[7] Perinatal mortality is the death of fetuses or neonates during the perinatal period. 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."
Comparison to fertilization age
+Comparison of dating systems for a typical pregnancy
! scope="col" Event
! width="200em" scope="col" Gestational age
(from the start of the last menstrual period)
! scope="col" Fertilization age
! scope="col" Implantation age Menstrual period begins Day 1 of pregnancy Not pregnant Not pregnant Has sex and Ovulation 2 weeks pregnant Not pregnant Not pregnant Fertilisation; 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 (or Day 0) Embryo stage begins; first missed period 4 weeks Day 15 Day 9 Cardiogenesis can be detected 5 weeks, 5 days Day 26 Day 20 Fetus 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
Estimation of due date
Medical fetal viability
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.(). What are the chances that my baby will survive?. ONLINE Available at: http://www.spensershope.org/chances_for_survival.htm . Last. 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
In classifying perinatal deaths, stillbirths and infant deaths
Postnatal use
Factors affecting pregnancy length
Heritability of pregnancy length
See also
External links
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