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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.
Conjoined twins, popularly referred to as Siamese twins, [1] [2] are twins joined in utero. [ a ] It is a very rare phenomenon, estimated to occur in anywhere between one in 50,000 births to one in 200,000 births, with a somewhat higher incidence in southwest Asia and Africa. [ 5 ]
[5] Vanishing twin syndrome has been characterized as the loss of a twin before 12 weeks of gestation, or early during the first trimester where it is uncommon for twin pregnancy to have been identified. [6] According to Boklage, most twins are ultimately born as singles, and vanished twins are a possible source of abnormal cells. [7]
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Heteropaternal superfecundation is common in animals such as cats and dogs. Stray dogs can produce litters in which every puppy has a different sire. Though rare in humans, cases have been documented. In one study on humans, the frequency was 2.4% among dizygotic twins whose parents had been involved in paternity suits. [6]
Large and giant breeds average 7 puppies per litter but can have a maximum litter size of about 15. [25] In one study, the Rhodesian Ridgeback had the highest average litter size with 8.9 pups per litter while the Pomeranian and Toy Poodle had the lowest with 2.4 pups per litter. [25] The number of puppies also varies with the mother's age.
The two lions are babies. They were born four weeks ago, making them newborn cubs. There's a reason why the two lion cubs had to wait a month before seeing the vet.
A multiple birth is the culmination of one multiple pregnancy, wherein the mother gives birth to two or more babies. A term most applicable to vertebrate species, multiple births occur in most kinds of mammals, with varying frequencies. Such births are often named according to the number of offspring, as in twins and triplets.