Gestation
Encyclopedia
Gestation is the carrying of an embryo
or fetus
inside a female
viviparous animal
. Mammals during pregnancy
can have one or more gestations at the same time (multiple gestations
).
The time interval of a gestation plus two weeks is called gestation period
, and the length of time plus two weeks that the offspring have spent developing in the uterus is called gestational age
. (The extra two weeks is added because gestational age is counted starting from the last menstrual period (LMP), rather than from actual conception. However, these extra two weeks are not always added when talking about animals.)
In humans, birth normally occurs at a gestational age of about 40 weeks, though a normal range is from 37 to 42 weeks.
Reminding
zygote
implants in the female's uterus
and ends once the fetus leaves the uterus.
Below are average and approximate values ordered by gestation period (note for humans gestational age is counted from the LMP, for other animals the counting method varies, so these figures could be 14 days off):
is an animal employing vivipary: the embryo develops inside the body of the mother, as opposed to outside in an egg
(ovipary). The mother then gives live birth
. The less developed form of vivipary is called ovoviviparity
, which, for instance, occurs in most vipers
. The more developed form of vivipary is called placental viviparity
; mammals are the best example, but it has also evolved independently in other animals, such as in scorpion
s, some shark
s, and in velvet worm
s. 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 animal
s develop within egg
s 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 embryo
s are nourished by the egg yolk
rather than by the mother's body. However, the mother's body does provide gas exchange
. Ovoviviparity is employed by many aquatic life forms such as fish
and some shark
s, reptile
s, and invertebrate
s. The young of ovoviviparous amphibians are sometimes born as larva
e, and undergo metamorphosis outside the body of the mother.
The Syngnathidae
family of fish 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 seahorse
s, the pipefish
, and the weedy
and leafy sea dragon
s. Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy
" has been applied.
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
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...
inside a female
Female
Female is the sex of an organism, or a part of an organism, which produces non-mobile ova .- Defining characteristics :The ova are defined as the larger gametes in a heterogamous reproduction system, while the smaller, usually motile gamete, the spermatozoon, is produced by the male...
viviparous animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
. Mammals during pregnancy
Pregnancy (mammals)
In mammals, pregnancy is the period of reproduction during which a female carries one or more live offspring from implantation in the uterus through gestation. It begins when a fertilized zygote implants in the female's uterus; and ends once it leaves the uterus.-Fertilization and implantation:A...
can have one or more gestations at the same time (multiple gestations
Multiple birth
A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets...
).
The time interval of a gestation plus two weeks is called gestation period
Gestation period
For mammals the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth. The duration of this period varies between species.-Duration:...
, and the length of time plus two weeks that the offspring have spent developing in the uterus is called gestational age
Gestational age
Gestational age relates to the age of an embryo or fetus . 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....
. (The extra two weeks is added because gestational age is counted starting from the last menstrual period (LMP), rather than from actual conception. However, these extra two weeks are not always added when talking about animals.)
Humans
Human pregnancy can be divided into three trimesters, each three months long. The first trimester is from the last period to the 13th week, the second trimester is from the 14th to 27th week, and the third trimester is from the 28th week to 42 weeks.In humans, birth normally occurs at a gestational age of about 40 weeks, though a normal range is from 37 to 42 weeks.
Reminding
Mammals
In mammals, pregnancy begins when a fertilizedFertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
implants in the female's uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
and ends once the fetus leaves the uterus.
Below are average and approximate values ordered by gestation period (note for humans gestational age is counted from the LMP, for other animals the counting method varies, so these figures could be 14 days off):
| Animal | Average gestation period (days) |
|---|---|
| American opossum | 12 - 13 |
| Golden hamster Golden Hamster The golden hamster or Syrian hamster, Mesocricetus auratus, is a very well known member of the rodent subfamily Cricetinae, the hamsters. In the wild they are now considered vulnerable. Their numbers have been declining due to loss of habitat and deliberate destruction by humans. However, they are... |
15 - 17 |
| House mice | 18 - 20 |
| Rabbit Rabbit Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world... s |
33 |
| Fox Fox Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to... es |
51 - 63 |
| Cat Cat The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests... s |
62 |
| Dog Dog The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in... s |
65 |
| Coati Coati Coatis, genera Nasua and Nasuella, also known as the Brazilian aardvark, Mexican tejón, hog-nosed coon, pizotes, crackoons and snookum bears, are members of the raccoon family . They are diurnal mammals native to South America, Central America, and south-western North America... s |
76-78 |
| Leopard Leopard The leopard , Panthera pardus, is a member of the Felidae family and the smallest of the four "big cats" in the genus Panthera, the other three being the tiger, lion, and jaguar. The leopard was once distributed across eastern and southern Asia and Africa, from Siberia to South Africa, but its... s |
94 |
| Lion Lion The lion is one of the four big cats in the genus Panthera, and a member of the family Felidae. With some males exceeding 250 kg in weight, it is the second-largest living cat after the tiger... s |
108 |
| Pig Pig A pig is any of the animals in the genus Sus, within the Suidae family of even-toed ungulates. Pigs include the domestic pig, its ancestor the wild boar, and several other wild relatives... s |
115 |
| Beaver Beaver The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world... s |
122 |
| Sheep | 145 |
| Baboon Baboon Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger... s |
187 |
| Moose Moose The moose or Eurasian elk is the largest extant species in the deer family. Moose are distinguished by the palmate antlers of the males; other members of the family have antlers with a dendritic configuration... |
245 |
| Human Human Humans are the only living species in the Homo genus... s |
259–294 |
| Cattle Cattle Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius... |
283 |
| Horse Horse The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today... s |
336 |
| Tapir Tapir A Tapir is a large browsing mammal, similar in shape to a pig, with a short, prehensile snout. Tapirs inhabit jungle and forest regions of South America, Central America, and Southeast Asia. There are four species of Tapirs: the Brazilian Tapir, the Malayan Tapir, Baird's Tapir and the Mountain... s |
380-400 |
| Giraffe Giraffe The giraffe is an African even-toed ungulate mammal, the tallest of all extant land-living animal species, and the largest ruminant... s |
420-450 |
| Sperm whale Sperm Whale The sperm whale, Physeter macrocephalus, is a marine mammal species, order Cetacea, a toothed whale having the largest brain of any animal. The name comes from the milky-white waxy substance, spermaceti, found in the animal's head. The sperm whale is the only living member of genus Physeter... s |
480–590 |
| Rhino Rhinoceros Rhinoceros , also known as rhino, is a group of five extant species of odd-toed ungulates in the family Rhinocerotidae. Two of these species are native to Africa and three to southern Asia.... s |
487 |
| Elephant Elephant Elephants are large land mammals in two extant genera of the family Elephantidae: Elephas and Loxodonta, with the third genus Mammuthus extinct... s |
600–660 |
Non-mammals
A viviparous animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
is an animal employing vivipary: the embryo develops inside the body of the mother, as opposed to outside in an egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
(ovipary). The mother then gives 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...
. The less developed form of vivipary is called ovoviviparity
Ovoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...
, which, for instance, occurs in most vipers
Viperidae
The Viperidae are a family of venomous snakes found all over the world, except in Antarctica, Australia, Ireland, Madagascar, Hawaii, various other isolated islands, and above the Arctic Circle. All have relatively long, hinged fangs that permit deep penetration and injection of venom. Four...
. The more developed form of vivipary is called placental viviparity
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
; mammals are the best example, but it has also evolved independently in other animals, such as in scorpion
Scorpion
Scorpions are predatory arthropod animals of the order Scorpiones within the class Arachnida. They have eight legs and are easily recognized by the pair of grasping claws and the narrow, segmented tail, often carried in a characteristic forward curve over the back, ending with a venomous stinger...
s, some shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, and in velvet worm
Velvet worm
The velvet worms are a minor ecdysozoan phylum. These obscurely segmented organisms have tiny eyes, antennae, multiple pairs of legs and slime glands. They have variously been compared to worms with legs, caterpillars and slugs...
s. 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 animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s develop within egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
s that remain within the mother's body up until they hatch or are about to hatch. This strategy of birth
Childbirth
Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...
is known as ovoviviparity. It is similar to vivipary
Vivipary
Vivipary has two different meanings. In animals, it means development of the embryo inside the body of the mother, eventually leading to live birth, as opposed to laying eggs...
in that the embryo develops within the mother's body. Unlike the embryos of viviparous species, ovoviviparous embryo
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...
s are nourished by the egg yolk
Egg yolk
An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...
rather than by the mother's body. However, the mother's body does provide gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
. Ovoviviparity is employed by many aquatic life forms such as fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and some shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s, and invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s. The young of ovoviviparous amphibians are sometimes born as larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e, and undergo metamorphosis outside the body of the mother.
The Syngnathidae
Syngnathidae
Syngnathidae is a family of fish which includes the seahorses, the pipefishes, and the weedy and leafy sea dragons. The name is derived from Greek, meaning "fused jaw" - syn meaning fused or together, and gnathus meaning jaws. This fused jaw trait is something the entire family has in common...
family of fish has the unique characteristic whereby females lay their eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
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 seahorse
Seahorse
Seahorses compose the fish genus Hippocampus within the family Syngnathidae, in order Syngnathiformes. Syngnathidae also includes the pipefishes. "Hippocampus" comes from the Ancient Greek hippos meaning "horse" and kampos meaning “sea monster”.There are nearly 50 species of seahorse...
s, the pipefish
Pipefish
Pipefishes or pipe-fishes are a subfamily of small fishes, which, together with the seahorses, form the family Syngnathidae.-Anatomy:...
, and the weedy
Weedy sea dragon
Phyllopteryx taeniolatus, the weedy seadragon or common seadragon, is a marine fish related to the seahorse. It is the only member of the genus Phyllopteryx. Adult weedy seadragons are a reddish colour, with yellow and purple markings; they have small leaf-like appendages that provide camouflage...
and leafy sea dragon
Leafy sea dragon
The leafy seadragon or Glauert's seadragon, Phycodurus eques, is a marine fish in the family Syngnathidae, which also includes the seahorses. It is the only member of the genus Phycodurus. It is found along the southern and western coasts of Australia. The name is derived from the appearance, with...
s. Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy
Male pregnancy
Male pregnancy refers to the incubation of one or more embryos or fetuses by male members of any species. In nearly all heterogamous animal species, offspring are ordinarily carried by the female until birth, but in fish of the Syngnathidae family , males perform this function...
" has been applied.
See also
- Evolution of mammalsEvolution of mammals__FORCETOC__The evolution of mammals within the synapsid lineage was a gradual process that took approximately 70 million years, beginning in the mid-Permian. By the mid-Triassic, there were many species that looked like mammals, and the first true mammals appeared in the early Jurassic...
(for the evolution of gestation-related features in humans and other mammals) - Male PregnancyMale pregnancyMale pregnancy refers to the incubation of one or more embryos or fetuses by male members of any species. In nearly all heterogamous animal species, offspring are ordinarily carried by the female until birth, but in fish of the Syngnathidae family , males perform this function...
- Nesting instinctNesting instinctNesting instinct refers to an instinct or urge in pregnant animals to prepare a home for the upcoming newborn. It is found in a variety of animals including humans.-In animals:...
- PregnancyPregnancyPregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
- Prenatal development
- Prenatal nutrition and birth weightPrenatal nutrition and birth weightNutrition and weight management before and during pregnancy has a profound effect on the development of infants. This is a rather critical time for healthy fetal development as infants rely heavily on maternal stores and nutrient for optimal growth and health outcome later in life. Prenatal...