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Drawing of a sagittal cross-section of a fetus in the pregnant parent's amniotic cavity.
Drawing of a fetus in utero.

Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent).[1] It is typical for mammals, but also occurs for some non-mammals. Mammals during pregnancy can have one or more gestations at the same time, for example in a multiple birth.

The time interval of a gestation is called the gestation period. In human obstetrics, gestational age refers to the fertilization age plus two weeks. This is approximately the duration since the pregnant parent's last menstrual period (LMP) began.


In mammals, pregnancy begins when a zygote (fertilized ovum) implants in the pregnant parent's uterus and ends once the fetus leaves the uterus during labor or an abortion (whether induced or spontaneous).

On the main article link above, are average and approximate gestation values ordered by number of days (note: human gestational age is counted from the last menstrual period; for other animals the counting method varies, so these figures could be 14 days off).


Timeline of human fertilization, ending with implantation of the blastocyst eight to nine days after fertilization.
Timeline of human fertilization

In humans, pregnancy can be defined clinically or biochemically. Clinically, pregnancy starts from the pregnant parent's last missed period. Biochemically, pregnancy starts when an individual's human Chorionic Gonadotropin (hCG) levels rise above 25 mIU/mL.[2]

Human pregnancy can be divided into three trimesters, each approximately three months long: the first, second, and third trimester. The first trimester is from the last menstrual period through the 13th week, the second trimester is 14th–27th week, and the third trimester is 28th–42nd week.[3] Birth normally occurs at a gestational age of about 40 weeks, though it is common for births to occur from 37 to 42 weeks.[3] Labor occurring prior to 37 weeks gestation is considered preterm labor and can result from multiple factors, including previous preterm deliveries.[4][5]

From the 9th week of pregnancy (11th week of gestational age), the embryo is called a fetus. Gestational age can be determined before or after birth. Before birth, health care providers will use an ultrasound to help determine the baby's age. They will measure the size of the baby's head, abdomen, and thigh bone. After birth, health care providers will measure the baby's weight, vital signs, reflexes, head circumference, muscle tone, and posture to help determine the gestational age.[6]

Various factors can influence the duration of gestation, including diseases in pregnancy.


Pregnant scorpion
Pregnant scorpion

In viviparous animals, the embryo develops inside the body of the mother, as opposed to outside in an egg (oviparity). The mother then gives live birth. The less developed form of viviparity is called ovoviviparity, which, for instance, occurs in most vipers. The more developed form of viviparity is called placental viviparity; mammals are the best example, but it has also evolved independently in other animals, such as in scorpions, some sharks, and in velvet worms. Viviparous offspring live independently and require an external food supply from birth. Certain lizards also employ this method such as the genera Tiliqua and Corucia. The placenta is attached directly to the mother in these lizards which is called viviparous matrotrophy.

Ovoviviparous animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch. This strategy of birth is known as ovoviviparity. It is similar to vivipary in that the embryo develops within the mother's body. Unlike the embryos of viviparous species, ovoviviparous embryos are nourished by the egg yolk rather than by the mother's body. However, the mother's body does provide gas exchange. The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother.

The fish family Syngnathidae has the unique characteristic whereby females lay their eggs in a brood pouch on the male's chest, and the male incubates the eggs. Fertilization may take place in the pouch or before implantation in the water. Included in Syngnathidae are seahorses, the pipefish, and the weedy and leafy sea dragons. Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.[7]

See also[edit]


  1. ^ "gestation | Definition of gestation in English by Oxford Dictionaries". Oxford Dictionaries | English.
  2. ^ "What is HCG?". American Pregnancy Association. 2020-04-26. Retrieved 2021-09-13.
  3. ^ a b American College of Obstetricians and Gynecologists - How Your Baby Grows During Pregnancy
  4. ^ Griggs, Kellie M.; Hrelic, Debra A.; Williams, Nina; McEwen-Campbell, Michelle; Cypher, Rebecca (November 2020). "Preterm Labor and Birth: A Clinical Review". MCN: The American Journal of Maternal/Child Nursing. 45 (6): 328–337. doi:10.1097/NMC.0000000000000656. ISSN 0361-929X.
  5. ^ "Preterm Labor and Birth". Retrieved 2021-09-13.
  6. ^ "Gestational age: MedlinePlus Medical Encyclopedia". Retrieved 2021-09-13.
  7. ^ Jones, Adam G.; Avise, John C. (2003-10-14). "Male Pregnancy". Current Biology. 13 (20): R791. doi:10.1016/j.cub.2003.09.045. PMID 14561416.

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