Gestational age
Gestational age relates to the age of an embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 or fetus
A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

 (or newborn infant). There is some ambiguity in how it is defined:
  • In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age.

  • In human obstetrics in many countries, gestational age is defined as the time elapsed since 14 days prior to conception. This is approximately the duration since the woman's last menstrual period (LMP) began. There is also a further distinction between the calendar gestational age described here, and the developmental gestational age determined by comparing an embryo or fetus to the average age of others that were at the same stage of development.

Unless the exact date of conception is known, counting from LMP has been the common method of computing gestational age. It involves the assumption that conception in humans typically occurs a consistent period (14 days) from the onset of the LMP. Although this "LMP method" of calculating gestational age is convenient, other methods are in use or have been proposed.


Some countries count gestational age from fertilization instead of LMP. This method of counting is also known as fertilization age, embryonic age, conceptional age or (intrauterine) developmental (IUD) age. This method is more prevalent in descriptions of prenatal development of the embryo or fetus. The LMP gestational age is usually greater by about two weeks. Also, pregnancy often is defined as beginning with implantation, which happens about three weeks after the LMP (see the beginning of pregnancy controversy
Beginning of pregnancy controversy
Controversy over the beginning of pregnancy usually occurs in the context of the abortion debate. Depending on where pregnancy is considered to begin, some methods of birth control or infertility treatment might be considered abortifacient...



Calculations of gestational age from LMP are sometimes incorrect due to normal variation from the average ovulation date. The gestational age of an individual infant can be more accurately estimated from:
  1. The mother
    A mother, mum, mom, momma, or mama is a woman who has raised a child, given birth to a child, and/or supplied the ovum that grew into a child. Because of the complexity and differences of a mother's social, cultural, and religious definitions and roles, it is challenging to specify a universally...

    's knowledge of the date of sexual intercourse
    Sexual intercourse
    Sexual intercourse, also known as copulation or coitus, commonly refers to the act in which a male's penis enters a female's vagina for the purposes of sexual pleasure or reproduction. The entities may be of opposite sexes, or they may be hermaphroditic, as is the case with snails...

  2. The mother's knowledge of fertility signs related to ovulation
    Ovulation is the process in a female's menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum . Ovulation also occurs in the estrous cycle of other female mammals, which differs in many fundamental ways from the menstrual cycle...

    . The needed observations are normally made by mothers who use fertility awareness
    Fertility awareness
    Fertility awareness refers to a set of practices used to determine the fertile and infertile phases of a woman's menstrual cycle. Fertility awareness methods may be used to avoid pregnancy, to achieve pregnancy, or as a way to monitor gynecological health....

     methods to get pregnant.
  3. Examination of the newborn infant. In the twentieth century, doctors (especially pediatricians) were trained to recognize the physical changes occurring to the fetus in the latter half of pregnancy so that a maturational age could be estimated.
  4. An obstetric ultrasound
    Obstetric ultrasonography
    Obstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...

     ("dating scan", in the UK routinely offered around 12 weeks) during the pregnancy, whereby sizes of certain fetal body parts are measured.

The fertilization age of children conceived by in vitro fertilization is known to the hour.


Using the LMP 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
Premature birth
In humans preterm birth refers to the birth of a baby of less than 37 weeks gestational age. The cause for preterm birth is in many situations elusive and unknown; many factors appear to be associated with the development of preterm birth, making the reduction of preterm birth a challenging...

 and consequently faces increased risk of morbidity and mortality
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....

. An estimated due date
Due Date
Due Date is a 2010 American comedy road film directed by Todd Phillips, co-written by Alan R. Cohen, Alan Freedland, and Adam Sztykiel, and starring Robert Downey, Jr. and Zach Galifianakis. The film was released on November 5, 2010...

 is given by Naegele's rule
Naegele's rule
Naegele's Rule is a standard way of calculating the due date for a pregnancy. The rule estimates the expected date of delivery by adding one year, subtracting three months, and adding seven days to the first day of a woman's last menstrual period...


The events of prenatal development usually occur at specific gestational ages. The gestational timing of a toxin exposure or infection can be used to predict the potential consequences to the fetus.

In classifying infant deaths and stillbirths

For most of the 20th Century, official definitions of a live birth
Live birth
In human reproduction, a live birth occurs when a fetus, whatever its gestational age, exits the maternal body and subsequently shows any sign of life, such as voluntary movement, heartbeat, or pulsation of the umbilical cord, for however brief a time and regardless of whether the umbilical cord or...

 and infant death
Infant mortality
Infant mortality is defined as the number of infant deaths per 1000 live births. Traditionally, the most common cause worldwide was dehydration from diarrhea. However, the spreading information about Oral Re-hydration Solution to mothers around the world has decreased the rate of children dying...

 in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 and Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

 differed from common international standards, such as those established by the World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 in the latter part of the century. Babies who were less than 28 weeks of gestational age, or weighed less than 1000 grams, or less 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.

It has been estimated that because of these special rules relying on gestational age as one criterion, the reported infant mortality rates in Russia and the Soviet Union were some 22 to 25 percent lower than they would have been had the WHO standard been applied.

It has been reported that Belgium and France have a practice of counting newborns of less than 26 weeks of gestational age that die shortly after birth as stillbirths.

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
Intraventricular hemorrhage
An intraventricular hemorrhage , often abbreviated "IVH," is a bleeding into the brain's ventricular system, where the cerebrospinal fluid is produced and circulates through towards the subarachnoid space...

 in preterms treated with extracorporeal membrane oxygenation
Extracorporeal membrane oxygenation
In intensive care medicine, extracorporeal membrane oxygenation is an extracorporeal technique of providing both cardiac and respiratory support oxygen to patients whose heart and lungs are so severely diseased or damaged that they can no longer serve their function...

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