Human Reproduction - Pregnancy and Embryonic Development

Following fertilization, the zygote undergoes mitotic divisions called cleavage as it moves through the oviduct toward the uterus, forming a $16$-celled embryo called a morula.

The morula continues to divide and transforms into a blastocyst. The blastocyst consists of an outer layer called the trophoblast and an inner cluster of cells called the inner cell mass.

The trophoblast layer becomes attached to the endometrium, a process known as implantation, which leads to pregnancy.

Chorionic villi and uterine tissue interdigitate to form the placenta, a structural and functional unit between the developing embryo (foetus) and the maternal body.

The placenta facilitates the supply of $O_2$ and nutrients to the embryo and the removal of $CO_2$ and excretory/waste materials produced by the embryo.

The placenta acts as an endocrine tissue and produces hormones like human chorionic gonadotropin ($hCG$), human placental lactogen ($hPL$), estrogens, and progesterones.

Hormones like $hCG$, $hPL$, and relaxin are produced in women only during pregnancy. Additionally, levels of other hormones like estrogens, progesterones, cortisol, prolactin, and thyroxine are increased several-fold in the maternal blood.

The inner cell mass contains certain cells called stem cells which have the potency to give rise to all the tissues and organs. It differentiates into an outer layer called ectoderm, an inner layer called endoderm, and a middle layer called mesoderm.

In humans, after one month of pregnancy, the embryo's heart is formed. By the end of the second month, the foetus develops limbs and digits.

By the end of $12$ weeks (first trimester), most of the major organ systems are formed, including external genitalia. Appearance of hair on the head and first movements are observed during the $5^{th}$ month.