Pregnancy, or gestation, occurs when one or more embryos develop in the uterus, and it is divided into three trimesters. During the first trimester, significant changes take place. After implantation, the hormone human chorionic gonadotropin (HCG) is secreted, which prevents the degradation of the corpus luteum and stops the menstrual cycle. HCG is also the hormone detected by most pregnancy tests.
The outer layer of the blastocyst, known as the trophoblast, grows into the endometrium to form the placenta, a vital organ that facilitates the exchange of materials, nutrients, and waste between the mother and fetus. The developing fetus is connected to the placenta via the umbilical cord, which contains two arteries and one vein. The embryo is classified as a fetus once it develops its adult structures in rudimentary form, a process known as organogenesis.
Humans are viviparous organisms, meaning we give birth to live offspring, unlike oviparous organisms that lay eggs. Some species exhibit ovoviviparity, where eggs remain inside the parent until they are ready to hatch. The process of labor involves uterine contractions that expel the fetus, stimulated by the hormone oxytocin. This hormone operates within a positive feedback loop: pressure from the fetus's head on the cervix triggers oxytocin release, which in turn stimulates more contractions, increasing pressure and further oxytocin release.
Humans are classified as eutherians due to our ability to give birth to well-developed offspring. Other mammals include monotremes, which lay eggs, and marsupials, which give birth to underdeveloped young that continue to develop in a pouch containing mammary glands. These mammary glands are a defining characteristic of mammals, secreting milk through a process called lactation. The hormone prolactin is responsible for milk production, while oxytocin facilitates milk secretion in response to the infant's suckling.