Every fetus goes through the same stages of development in its mother's womb. It starts as a single-celled zygote, becomes a blastocyst, then implants in the womb to become an embryo. Once embryonic development is complete, the developing person is referred to as a foetus.
Fertilisation occurs when a sperm cell from the male penetrates the female's egg cell. When this happens, the resulting cell is called a zygote and contains all the genetic material necessary to develop into a new human being. The zygotic cell begins to divide and eventually becomes a blastocyst, which is essentially a tiny ball of identical cells. The blastocyst drifts from the female's fallopian tubes into her uterus, where it implants in the fleshy, nutrient-rich wall there. Five days usually elapse between fertilisation and implantation. Once implanted, the blastocyst is referred to as an embryo.
Early embryonic period
During the early embryonic stage--three to five weeks after conception or weeks five to seven of pregnancy--the cells of the blastocyst differentiate and organs begin to form. In the initial embryonic stages, the brain, spinal cord, heart and gastrointestinal tract begin to develop. Then buds appear that will eventually elongate into arms and legs. In week seven of the pregnancy, the heart develops a regular beat, the eyes and ears begin to form and the bones of the vertebral column appear.
Late embryonic period
The late embryonic period extends from week eight to week 10 of the pregnancy. During this time, the nose, jaw, lungs and palate form. Fingers and toes appear at the end of the elongated arms and legs. The eyes move forward on the face and gain lids. By the end of week nine, all major organs have begun to develop. By the end of week 10, external ears and genitalia have appeared and the facial features have become more defined. The developing person's muscles are able to contract and circulation proceeds at a healthy pace. At this time, the embryo becomes fully recognisable as a human being, which marks the beginning of the foetal period.
Early foetal period
During the foetal period, the developing baby's organs become fully functional for the purpose of being able to function outside of the mother's womb. During the early foetal period--weeks 11 to 24 of the pregnancy--there's increased development. The bones continue to harden and the baby begins to make active movements, including sucking motions with the mouth. The baby can hear by week 19 or 20 of the pregnancy. During week 22, hair and other protein-based external structures, such as nails, appear. During weeks 23 to 25 the baby begins to make its own blood cells, the lungs develop air sacs and eyes and fingerprints have developed. By the end of the early foetal period, the baby's heartbeat can be heard and the mother can feel the baby's movements inside her.
Late foetal period
This final stage of maturation begins with rapid brain development around week 25 of the pregnancy and continues until the baby is born. The nervous system begins to exert control of the baby's body, the respiratory system begins to work, the bones are fully developed and the baby's body gains a lot of fat. By week 40, all of the baby's vital organ systems are prepared to function outside of the mother.