The gestation period is the length of time, in viviparous (live-bearing) animals) needed for a fetus to develop inside the body of its mother. Gestation begins with conception (the implanta­tion of the embryo in the uterus) and ends with labor and birth.

The length of the gestational period is species­specific, correlating somewhat with the body size for any given species, with larger species generally having a more prolonged gestational period. The gestational period is therefore quite short for very small mammals (21 days for mice), is longer for medium-sized animals (approximately 60 days for cats, dogs, and raccoons), and is quite lengthy for such large animals as rhinoceros (16 months) or whales (18 months). The shortest gestation period is that of the opossum (12 days), and the longest known gestation period for any animal is that of the elephant (18-22 months). For humans, the gestation period is normally 40 weeks (280 days), measured from the first day of the mother’s last menstrual period.

Neonate Type

Although the length of the gestation period is gen­erally correlated to an animal’s body size, it is also determined by the state of development at birth for offspring of that species. Among mammals, there are three different groups that can be defined by the level of gestational development of their offspring at birth, and these groupings have proven (through fossil evidence) to be remarkably consistent throughout evolutionary history. Marsupials, which have the shortest gestation periods of all animals, are considered “superaltri- cial,” with offspring that are very underdeveloped at birth and usually crawl into a pouch of some kind to continue their development before being able to survive detached from their mother’s body. A second group of mammals includes insectivores, carnivores, and rodents. They have somewhat longer gestational periods and give birth to “altri­cial” offspring, which are generally poorly devel­oped and typically must develop in a nest or burrow for some period after birth so that they can continue to grow and learn how to move around independently. Altricial offspring are still underdeveloped at birth, being usually hairless and with their eyes and ears sealed off by mem­branes. Altricial animals bear multiple offspring in litters and tend to have shorter gestation periods, reflecting the fact that the mother has limited space in her womb for completing development prior to birth. In contrast, a third group of mam­mals (primates, whales, dolphins, and hoofed mammals) has very long gestational periods and usually gives birth to only one or two well- developed “precocial” offspring that are relatively active soon after birth; they are born with fully developed vision and hearing, and are protected by fur and hair. This type of gestation is most com­mon in animals that live out in the open, exposed to the elements, and are unable to conceal their young in burrows or caves.

Each main mammal group is therefore charac­terized by the type of neonate (altricial, superaltri- cial, or precocial) that its members produce, and by the length of gestational period typical of that group. Insectivores, tree shrews, carnivores, and many rodents have short gestational periods and bear altricial offspring; hoofed mammals, hyraxes, elephants, cetaceans (whales), pinnipeds (walruses, seals, and sea lions), primates, and hystricomorph rodents (Old World porcupines) bear precocial offspring after a fairly long gestational period. In fact, phylogenetic groupings have more impact on the length of the gestational period than animal body size does; the gestation period for an animal that bears precocial offspring will typically be three times longer than that for an animal of the same body size that bears altricial offspring.

Gestational Age

The gestational age of a fetus is a measure of how far along it is in the gestational period. Since the exact time of conception is not always known, the beginning of gestation is usually dated from a par­ticular point in the reproductive cycle of any given species, or is estimated based on the size of the devel­oping fetus. In humans, for example, gestational age is projected by a measure of time elapsed since the first day of the mother’s last menstrual period, or is calculated with an ultrasound examination (usually administered between the 8th and 18th weeks of pregnancy), which uses the size of the developing fetus as the most accurate measure for dating con­ception and projecting when full development and birth will occur.

The gestational period is divided into three major phases: the preimplantation phase (from fertilization to implantation in the mother’s womb), an embryonic phase (from implantation to the formation of recognizable organs), and a fetal phase (from organ formation to birth). These phases are called trimesters. In humans, each phase lasts about 3 months; the first trimester lasts from weeks 1 through 12, the second from weeks 13 through 27, and the third from weeks 28 to 40. In some mammals, the gestation period is extended to include an additional phase known as delayed implantation, or embryonic diapause, that precedes the embryonic phase. In these instances, the fertilized egg develops into a blasto­cyst (forming a hollow ball of cells), but then it stops developing and remains free in the womb rather than implanting in the wall of the womb (the usual next step in gestational development for most mammals). Delayed implantation is a strat­egy used by animals to delay the birth of a fetus until such external variables as food or weather are favorable for the survival of offspring, or in some cases as a way of holding potential offspring in reserve to immediately replace older offspring that do not survive long after birth. Other factors that influence the length of the gestational period in minor ways include gender (males take a few days longer than females), the number of young (single young take a few days longer than multi­ples), and heredity through the maternal line. In general, though, there is very little variation in the length of the gestational period within the same species, that is, 4% or less variability.

Helen Theresa Salmon

See also Birthrates, Human; Clocks, Biological;

Duration; Evolution, Organic; Fertility Cycle;

Gestation Period; Life Cycle; Maturation

Further Readings

Assali, N. S. (1968). Biology of gestation: Vols. 1 and 2. Burlington, MA: Academic Press.

Davies, J. (1960). Survey of research in gestation and the developmental sciences. Baltimore, MD: Williams & Wilkins.

Gilbert, S. F. (2006). Developmental biology. Sunderland, MA: Sinauer Associates.

Grzimek, B., Schlager, N., & Olendorf, D. (2003). Life history and reproduction. In D. G. Kleiman, V. Geist, & M. C. McDade (Eds.), Grzimek’s animal life encyclopedia (Vol. 12, pp. 95-96). Farmington Hills, MI: Thomson Gale.