Cetaceans are born being able to swim, and maintain their body temperature. Their birth weight averages 10 to 15% of the mother's weight in dolphins. Most animals have only one young. The gestation period is 10 to 12 months, although sperm and pilot whales have a 16 month gestation.

Like most animals giving birth to only one young, cetaceans occasionally have twins. The percentage of twins varies from 0.39% in the humpback to 1.09% in the sei whale. These figures are similar to those for men, horses and cows. Multiplets have also been found, the highest being six.

The fetus may die while still in the uterus, as with many animals. The rate of prenatal mortality increases with the age of the mother. Young fetuses are resorbed, while older ones are mummified.

During pregnancy, the left uterine horn is almost always the one which contains the calf, while the right contains only part of the embryonic envelopes and the placenta. An observed average of only 17% of dolphins had the fetus in the right horn. Although both horns are identical, the left ovary seems to be more active than the right, and this may explain it. Even when the right ovary ovulates, the implantation seems to always occur in the left horn. This cross-over is not considered unusual.

The umbilical cord consists of two arteries, two veins, and an "allantonic" duct, imbedded within a gelatinous connective tissue, which is tougher and much more compact than that of man.

Most of the growth of the fetus is towards the side and front of the mother, reducing the visible external bulge. Between the seventh and eleventh month of pregnancy the fetus is aligned with it's tail toward the cervix. This allows the young to be born almost invariably tail-first.

Being born tail-first has an obvious advantage; it enables the young to breathe when not yet fully expelled, without the danger of breathing in nonsterile blood, mucus, or amniotic fluid in the maternal organs. This is important, since the birth may last from half an hour to over two hours. However, one bottlenose dolphin birth was reported to occur in only five minutes.

In terrestrial mammals the stimulus that begins breathing is thought to be a shortage of oxygen in the blood of the newborn. As long as sufficient oxygen can be supplied by the placenta, there is no danger that the young will be forced to breathe. However, should the cord become taut, get a knot or impede the blood flow in any other way, the young may be forced to breathe early. In many land mammals the cord is fairly short, about 40% of the fetus' length. It is stretched taut at the moment the forehead of the young protudes from the mother. (Assuming the young is born head first. Because the cord is located closer to the rear of the animal than the snout, an animal born in breech position may have the cord taut or even snap before full born, causing the onset of breathing at a dangerous moment.)

In cetaceans, the umbilical cord ranges from 40 to 50% of the fetus' length, but is located in the approximate center of the body. Thus, there is no real preference for head or tail presentation. Because the young are born under water, the risk of breathing in fluid will always be the same.

The flippers and flukes are folded at birth in such a way that they are not likely to catch on the mother's pelvic bone during birth, and as such are not a worry.

The reason the cetacean is always born tail first is due to distribution of weight in the fetus. Since the tail is the lighter part of the fetus, it will rise to a higher point than the head and body. Due to the layout of the uterus, the highest point leads towards the cervix. In most terrestrial mammals, the opposite is true, the hindquarters tend to be heavier, and the head will point towards the opening of the cervix.

The umbilical cord has a weak point which allows it to snap of it's own accord as the newborn is expelled. The forced needed to tear the cord is extremely little, in many cases it cannot support even it's own weight.

In the final stages of pregnacy the fetal movements are easily observed. As term approaches, the female's number of respirations and defecations increases to an almost continuous level. With the onset of labour swimming is slowed almost to a stop. The first labour pains, caused by uterine contractions, may last from thirty to sixty minutes.

When the young is completely expelled and the umbilical cord is broken the newborn immediately swims to the surface to breathe. If it fails to do so, the mother will push it up to the surface, even if it is stillborn. The afterbirth is expelled several hours later, and is ignored.

In wild dolphins, a second female dolphin oftens attends to a pregnant dolphin, and may assist with the delivery and helping the newborn to the surface. At birth, the remainder of the pod usually forms a protective circle around the mother to protect her and ward off predators who may be attracted by the blood.

When suckling, the young approach from the rear. The mother will often turn to her side, to aid the youngster in finding the nipple. As the young lacks proper lips it may not be able to grasp the nipple firmly. It is believed the young makes a "tube" with it's tongue and the roof of it's mouth. The mother then "squirts" the milk into the young's mouth. This speeds feeding, since the young cannot suckle for long periods due to the necessity to breathe.

Cetacean milk is creamy white in colour, and may be tinted pink. It contains an unusually high percentage of fat, and a low sugar content. This may be due to the fact that the animal must develop quite quickly. A blue whales doubles it's birth weight in only 7 days, while a cow may take 50.

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