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.

Sometimes, a zygote can be created by humans outside of the animal's body in the artificial process of in-vitro fertilization.

Then, a flat layer cell forms on the exterior of this cavity, and the zona pellucida, the blastocyst's barrier, remains the same size as before.

The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation.

At the same time, the bottom layer forms a small sac (if the cells begin developing in an abnormal position, an ectopic gestation may also occur at this point).

Its growth makes the embryo undergo gastrulation, in which the three primary tissue layers of the fetus, the ectoderm, mesoderm, and endoderm, develop.

The mesoderm forms muscle, bone, and lymph tissue, as well as the interior of the lungs, heart, and reproductive and excretory systems.

The ectoderm forms the skin, nails, hair, cornea, lining of the internal and external ear, nose, sinuses, mouth, anus, teeth, pituitary gland, mammary glands, eyes, and all parts of the nervous system.

For mammals, the gestation period is the time in which a fetus develops, beginning with fertilization and ending at birth.

For most species, the amount a fetus grows before birth determines the length of the gestation period.

Species that use a breeding season usually give birth during a specific time of year when food is available.