Gestational age
In
Gestational age is contrasted with
Methods
According to American College of Obstetricians and Gynecologists, the main methods to calculate gestational age are:[2]
- Directly calculating the days since the beginning of the last menstrual period
- Early 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.[2]
- In case of co-incubation and adding 14 days.[3]
Gestational age can also be estimated by calculating days from
A more complete listing of methods is given in following table:[5]
Method of estimating gestational age | Variability (2 standard deviations)[5] |
---|---|
Days from 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 (crown-rump length) | ±8% of the estimate |
Second-trimester obstetric ultrasonography (head circumference, femur length) | ±8% of the estimate |
Third-trimester obstetric ultrasonography (head circumference, femur length) | ±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.[5] 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.[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.[6]
Assessment of gestational age can be made based on selected head and trunk parameters.
-
By gestational sac diameter
-
By crown-rump length (CRL)
-
Bybiparietal diameter(BPD)
Comparison to fertilization age
The fertilization or conceptional age (also called embryonic age and later fetal 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 (LMP).[8] 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.[9] In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days.[8] 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,[10] and may be mistaken as menstruation, potentially giving an estimated gestational age that is too low.
Uses
Gestational age is used for example for:[citation needed]
- 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
The most common system used among healthcare professionals is
Medical fetal viability
There is no sharp limit of development, gestational age, or weight at which a human fetus automatically becomes viable.
Completed weeks of gestation at birth | 21 and less | 22 | 23 | 24 | 25 | 26 | 27 | 30 | 34 |
---|---|---|---|---|---|---|---|---|---|
Chance of survival[15] | <1%[17] | 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[18] |
37 0/7 - 38 6/7 | Early Term[19] |
39 0/7 - 40 6/7 | Full Term[19] |
41 0/7 - 41 6/7 | Late Term[19] |
> 42 0/7 | Postterm |
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
According to the WHO, a preterm birth is defined as "babies born alive before 37 weeks of pregnancy are completed."[20] 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).[20] Various jurisdictions may use different classifications.
In classifying perinatal deaths, stillbirths and infant deaths
For most of the 20th century, official definitions of a live birth and infant death in the Soviet Union and Russia differed from common international standards, such as those established by the World Health Organization in the latter part of the century.[21][22] Babies who were fewer than 28 weeks of gestational age, or weighed fewer than 1000 grams, or fewer than 35 cm in length – even if they showed some sign of life (breathing, heartbeat, voluntary muscle movement) – were classified as "live fetuses" rather than "live births." Only if such newborns survived seven days (168 hours) were they then classified as live births. If, however, they died within that interval, they were classified as stillbirths. If they survived that interval but died within the first 365 days they were classified as infant deaths.
More recently, thresholds for "
The WHO defines the
Postnatal use
Gestational age (as well as fertilization age) is sometimes used postnatally (after birth) to estimate various risk factors. For example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation.[27]
Factors affecting pregnancy length
Child's gestational age at birth (pregnancy length) is associated with various likely causal maternal non-genetic factors: stress during pregnancy,
Heritability of pregnancy length
Family-based studies showed that gestational age at birth is partially (25–40%) determined by genetic factors.[31]
See also
- Pregnancy
- Maternity
- Prenatal development
- Gestation periods in mammals
- Abortion law
- Reproductive rights
- Fetal rights
References
- ^ "Gestational age: MedlinePlus Medical Encyclopedia". medlineplus.gov.
- ^ American Congress of Obstetricians and Gynecologists. Created November 2012.
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- ^ a b Moore, Keith and Persaud, T. The Developing Human: Clinically Oriented Embryology, p. 103 (Saunders 2003).
- ^ March of Dimes --> Neonatal Death Archived 2014-10-24 at the Wayback Machine Retrieved on November 10, 2014. In turn citing:
- Tyson JE, Parikh NA, Langer J, Green C, Higgins RD (April 2008). "Intensive care for extreme prematurity--moving beyond gestational age". N. Engl. J. Med. 358 (16): 1672–81. PMID 18420500.
- Luke B, Brown MB (December 2006). "The changing risk of infant mortality by gestation, plurality, and race: 1989-1991 versus 1999-2001". Pediatrics. 118 (6): 2488–97. PMID 17142535.
- The American College of Obstetricians and Gynecologists (September 2002). "ACOG Practice Bulletin: Clinical Management Guidelines for Obstetrcian-Gynecologists: Number 38, September 2002. Perinatal care at the threshold of viability". Obstet Gynecol. 100 (3): 617–24. PMID 12220792.
- Tyson JE, Parikh NA, Langer J, Green C, Higgins RD (April 2008). "Intensive care for extreme prematurity--moving beyond gestational age". N. Engl. J. Med. 358 (16): 1672–81.
- ^ a b (). What are the chances that my baby will survive?. [ONLINE] Available at: http://www.spensershope.org/chances_for_survival.htm Archived 2018-08-09 at the Wayback Machine. [Last Accessed 14 November 2012].
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- ^ "World's most premature baby defies sub-1% survival odds to break record". Guinness World Records. 10 November 2021. Retrieved 2022-03-15.
- ^ Late-Preterm Infants Archived 2012-05-02 at the Wayback Machine ACOG Committee Opinion 404
- ^ a b c Ob-Gyns Redefine Meaning of "Term Pregnancy" Archived 2017-05-03 at the Wayback Machine, from American College of Obstetricians and Gynecologists. October 22, 2013
- ^ a b [1][full citation needed]
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- ^ Anderson, Barbara A.; Silver, Brian D. (1994). "The Geodemography of Infant Mortality in the Soviet Union, 1950-1990". PSC Research Report No. 94-316: 8.
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- ^ Li, Z; Zeki, R; Hilder, L; Sullivan, EA (2012). "Australia's Mothers and Babies 2010". Perinatal statistics series no. 27. Cat. no. PER 57. Australian Institute of Health and Welfare National Perinatal Statistics Unit, Australian Government. Retrieved 4 July 2013.
- ^ Royal College of Obstetricians; Gynaecologists UK (April 2001). "Further Issues Relating to Late Abortion, Fetal Viability and Registration of Births and Deaths". Royal College of Obstetricians and Gynaecologists UK. Archived from the original on 5 November 2013. Retrieved 4 July 2013.
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