[Biology Class Notes] on Gestation Pdf for EXAM

In mammals, Gestation (or the gestation period definition) is defined as the time between conception and birth, during which the fetus or embryo is developing in the uterus. This particular definition raises occasional difficulties because, in a few species (for example, man and monkeys), the exact time of conception cannot be known. In such cases, the gestation beginning is usually dated from a few well-defined points in the reproductive cycle (for example, the beginning of the previous menstrual period).

About Gestation

The length of gestation differs from species to species. The shortest gestation period, which is known is that of the Virginia opossum, up to 12 days, whereas the longest that of the Indian elephant is up to 22 months. Whereas, in the course of evolution, the gestation duration has become adapted to the species’ needs. The degree of ultimate growth is a major factor, and usually, the smaller animals have shorter gestation periods than the larger ones. Exceptions are given as the guinea pig and other related South American rodents, where the gestation is prolonged (on an average of 111 days for the chinchilla and 68 days for the guinea pig).

The young of the given species are born in a greater maturity state than are the ones of the rat with its period of 22 days. The other factor is given as, in several species with restricted breeding seasons; gestation can be adjusted so that birth coincides with the period whenever the food is most abundant. Therefore, a spring breeder, the horse with 11 months of gestation, has its young the following spring, as the sheep does, a fall breeder having five months of gestation. Animals that live in the open space tend to have longer gestations and to bear the young ones that have reached a greater maturity state than do animals that may conceal their young in caves or the underground burrows. Generally, the Marsupials have short gestations.

For example, it takes 40 days for the largest kangaroos. The young, which is born in an extremely immature state, transfers to the pouch, where the gestation can be said to continue.

Embryos of a few species experience an arrest in the development that prolongs the gestation greatly. This is especially true of the carnivores of fur-bearing ones, the weasels and martens. Embryos of the American marten and European badger, which breed in July and August, develop for some days, then they lie dormant in the uterus, being implanted in January, and the birth takes place in March. Of the total 250 days of the gestation period, growth takes place during only 50 days. And, the delayed implantation also takes place in mice and some other small rodents that become pregnant while still, they are suckling a litter.

Determination of Length of Gestation

Either a great number of minor factors or a simple single factor, all culminating at a date or to the near one date, determine the gestation length. Many minor variations are the known ones: in men, gestation for males ranges from three to four days longer than that for the females, whereas in cattle, bulls are carried up to one day longer compared to the heifers. In both the species gestation of twins, gestation is up to five to six days less than for singlets.

Let us look at the gestation period of animals.

In the case of animals such as the pig or rabbit, which bear several young at a time, the gestation period is shorter for larger litters compared to the smaller ones.

Also, heredity influences the gestation factor; in cattle, the mean gestation period for the Holstein-Friesians is given as 279 days; for Brown Swiss, it is up to 290 days; whereas, the other breeds fall between these extremes. And, when hybrids are produced by crossing the two species with variable gestation periods, the hybrid one is carried for a period lying somewhere between the two parents and tending toward the species of the mother. Therefore, a mare carries a mule foal (which is fathered by a jackass) up to 10 days longer compared to the normal period for the horse (which is up to 337 days). For human gestation, the longest or shortest gestation period for humans depends on the pregnancy.

Gestational Age

Generally, the exact date of conception is not known for humans. Hence, gestational age is the common way to measure how far along any pregnancy is. Where the baby is in development – such as whether the toes and fingers have formed – is tied to the gestational age.

Gestational age can be measured in weeks from the first day of the last menstrual period. It means that the last period counts as part of pregnancy. Even though the person is not actually pregnant, the period is a signal that the body is preparing for pregnancy.

Fetal growth does not generally start until conception, which is when the sperm fertilizes an egg.

Also, the doctor can determine the gestational age with the help of ultrasound or after delivery.

During an ultrasound scanning, the doctor will measure the baby’s head and the pregnant lady’s abdomen to determine the gestational age.

After the birth takes place, the gestational age is defined using the Ballard Scale, which assesses the baby’s physical maturity.

We can divide the Gestational age into two periods as embryonic and fetal.

The embryonic period is given as the 5th week of pregnancy, which means when the embryo implants in the females’ uterus – to the 10th week. On the other side, the fetal period is the 10th week of birth.