Gestational age

Gestational age relates to the age of an embryo or fetus (or newborn infant). There is some ambiguity as to how it is defined:


 * In embryology, the term "gestational age" is seldom used because it lacks precision. The timing of embryonic development starts with fertilization. In mammals, this proceeds through uterine implantation, until birth.


 * In human obstetrics, gestational age is often defined as the time elapsed since 14 days prior to fertilization. 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 fertilization is known, counting from LMP has been the common method of computing gestational age. It involves the assumption that fertilization 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.

Methods
Some countries count gestational age from fertilization instead of LMP. This method of counting is also known as fertilization age, embryonic age, fertilizational 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).

Accuracy
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 woman's knowledge of the date of sexual intercourse.
 * 2) The woman's knowledge of fertility signs related to ovulation. The needed observations are normally made by women who use fertility awareness 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 ("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.

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

Uses
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 and consequently faces increased risk of morbidity and mortality. An estimated due date is given by Naegele's rule.



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 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. 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 in preterms treated with extracorporeal membrane oxygenation.