Pregnancy and embryonic development

After implantation, finger-like chorionic villi grow out of the trophoblast and are surrounded by uterine tissue and maternal blood. The villi and uterine tissue interlock to form the placenta, the structural and functional link between embryo and mother.

The placenta supplies oxygen and nutrients to the embryo and removes carbon dioxide and excretory wastes. It is connected to the embryo by the umbilical cord, which carries substances to and from the embryo.

The placenta is also an endocrine tissue, producing hCG (human chorionic gonadotropin), hPL (human placental lactogen), estrogens and progestogens, among others. Later in pregnancy the ovary secretes relaxin.

hCG, hPL and relaxin are found in a woman only while she is pregnant. Pregnancy also raises the maternal blood levels of estrogens, progestogens, cortisol, prolactin and thyroxine several times over; this hormonal rise sustains the growing foetus, brings about metabolic changes in the mother and keeps the pregnancy going.

Right after implantation, the inner cell mass differentiates into three layers: outer ectoderm, middle mesoderm and inner endoderm, which together give rise to all tissues and organs of the adult.

The inner cell mass contains stem cells, which can give rise to every tissue and organ.

Human pregnancy lasts about 9 months. The heart forms by the end of the first month, and the heartbeat can be heard with a stethoscope as the first sign of a growing foetus. Limbs and digits appear by the end of the second month.

Most major organ systems, including well-developed limbs and external genital organs, have formed by week 12, the end of the first trimester.

The first foetal movements are felt in the fifth month, when hair also appears on the head. By the end of about 24 weeks (second trimester), fine hair covers the body, the eyelids separate and eyelashes form. At the end of nine months the foetus is fully developed and ready for delivery.