`=>` `color{blue}"Chorionic villi"`: Theses are the `color{Violet}"numerous branching projections"` from the external surface of the chorion that provide for exchange between the `color{Violet}"maternal and fetal circulation"`. `color{Violet}"Oxygen and nutrients"` in the maternal blood diffuse through the walls of the villi and enter the blood of the embryo or fetus. `color{Violet}"Carbon dioxide and waste"` products diffuse from blood in the fetal capillaries through the walls of the villi into the maternal blood.

`=>` `color{blue}"Placenta"` : The placenta is an organ that connects the `color{Violet}"developing fetus to the uterine wall"` to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. The placenta provides `color{Violet}"oxygen and nutrients"` to growing fetuses and removes `color{Violet}"waste products"` from the fetus's blood. It comes from Greek plakóenta/plakoúnta, meaning `color{Violet}"flat, slab-like"`, in reference to its round, flat appearance in humans.

`=>` `color{blue}"Embryonic Layers"`: An `color{Violet}"Embryonic layer (germ layer)"` is a primary layer of cells that form during embryogenesis from which the `color{Violet}"endoderm, mesoderm, and ectoderm"` arise. From these layers of cells arise all of the `color{Violet}"structures and organs"` and parts of the body. The `color{Violet}"endoderm and ectoderm"` are the first to develop.

`=>` `color{blue}"Ectoderm"`: `color{Violet}"Ectoderm"` is the the outermost of the three germ layers, or masses of cells, which appears early in the development of an `color{Violet}"animal embryo"`. In vertebrates, ectoderm subsequently gives rise to `color{Violet}"hair, skin, nails or hooves"`, and the lens of the eye; the `color{Violet}"epithelia"` (surface, or lining, tissues) `color{Violet}"of sense organs"`, the nasal cavity, the sinuses, the `color{Violet}"mouth (including tooth enamel)"`, and the anal canal; and nervous tissue.

`=>` `color{blue}"Endoderm"`: `color{Violet}"Endoderm"`, the innermost of the three germ layers, or masses of cells (lying within ectoderm and mesoderm), which appears early in the development of an `color{Violet}"animal embryo"`. The endoderm subsequently gives rise to the epithelium (tissue that covers, or lines, a structure) of the `color{Violet}"pharynx"`, including the eustachian tube, the tonsils, the thyroid gland, parathyroid glands, and thymus gland; the `color{Violet}"larynx, trachea, and lungs"`; the `color{Violet}"gastrointestinal tract"` (except mouth and anus), the `color{Violet}"urinary bladder, the vagina"` (in females), and the urethra.

`=>` `color{blue}"Mesoderm"`: `color{Violet}"Mesoderm"`, the middle of the three germ layers, or masses of cells (lying between the ectoderm and endoderm), which appears early in the development of an `color{Violet}"animal embryo"`. In vertebrates it subsequently gives rise to `color{Violet}"muscle, connective tissue,"` `color{Violet}"cartilage, bone, notochord, blood,"` bone marrow, lymphoid tissue, and to the epithelia (surface, or lining, tissues) of blood vessels, lymphatic vessels, body cavities, kidneys, ureters, gonads (sex organs), genital ducts, adrenal cortex, and certain other tissues.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● After implantation, `color{Violet}"finger-like projections"` appear on the trophoblast called `color{Violet}"chorionic villi"` which are surrounded by the uterine tissue and maternal blood.

● The `color{Violet}"chorionic villi"` and `color{Violet}"uterine tissue"` become `color{Violet}"interdigitated"` with each other and jointly form a `color{Violet}"structural and"` `color{Violet}"functional unit"` between developing embryo (fetus) and maternal body called `color{Violet}"placenta"` .

● The `color{Violet}"placenta"` facilitates the `color{Violet}"supply"` of oxygen and nutrients to the embryo and also `color{Violet}"removal"` of carbon dioxide and excretory/waste materials produced by the embryo.

● The placenta is connected to the embryo through an `color{Violet}"umbilical cord"` which helps in the `color{Violet}"transport"` of substances to and from the embryo.

● Placenta also acts as an endocrine tissue and produces several hormones like `color{Violet}"human chorionic gonadotropin (hCG)"`, `color{Violet}"human placental lactogen (hPL)"`, `color{Violet}"estrogens"`, `color{Violet}"progestogens"`, etc.

● In the `color{Violet}"later phase"` of pregnancy, a hormone called `color{Violet}"relaxin"` is also secreted by the ovary.

● Thus, `color{Violet}"hCG, hPL"` `color{Violet}"and relaxin"` are produced in women only during `color{Violet}"pregnancy"`.

● In addition, during pregnancy the `color{Violet}"levels"` `color{Violet}"of other hormones"` like estrogens, progestogens, cortisol, prolactin, thyroxine, etc., are `color{Violet}"increased"` several folds in the maternal blood.

● `color{Violet}"Increased production"` of these hormones is essential for supporting the `color{Violet}"fetal growth"`, `color{Violet}"metabolic changes"` in the mother and `color{Violet}"maintenance"` of pregnancy.

● Immediately after implantation, the `color{Violet}"inner cell mass"` (embryo) differentiates into an outer layer called `color{Violet}"ectoderm"` and an inner layer called `color{Violet}"endoderm"`.

● A `color{Violet}"mesoderm"` soon appears between the ectoderm and the endoderm.

● These `color{Violet}"three layers"` give rise to all tissues (organs) in adults.

● The `color{Violet}"inner cell mass"` contains certain cells called `color{Violet}"stem cells"` which have the potency to give rise to all the `color{Violet}"tissues and organs"`.

● The `color{Violet}"human pregnancy"` lasts `color{Violet}"9 months"`.

● The `color{Violet}"first sign"` of growing foetus may be noticed by listening to the `color{Violet}"heart sound"` carefully through the stethoscope.

`star` Stages in `color{Violet}"Prenatal Development"`:
● `color{Brown}"After one month of pregnancy"`: The `color{Violet}"embryo’s heart"` is formed.

● `color{Brown}"End of the second month of pregnancy"`: The fetus develops `color{Violet}"limbs and digits"`.

● `color{Brown}"End of 12 weeks (first trimester)"`: Most of the `color{Violet}"major organ systems"` are formed, for example, the `color{Violet}"limbs"` and `color{Violet}"external genital organs"` are well-developed.

● `color{Brown}"During the fifth month"`: The `color{Violet}"first movements"` of the fetus and appearance of `color{Violet}"hair on the head"` are usually observed.

● `color{Brown}"End of 24 weeks (second trimester)"`: The body is covered with `color{Violet}"fine hair"`, `color{Violet}"eye-lids separate"`, and `color{Violet}"eye lashes"` are formed.

● `color{Brown}"End of nine months of pregnancy"`: The fetus is `color{Violet}"fully developed"` and is ready for delivery.

`=>` `color{blue}"Parturition"` : Also called `color{Violet}"labor and delivery"`. Parturition comes from the Latin parturire, `color{Violet}"to be ready to bear young"`. It is the process of delivering the baby and placenta from the uterus through the vagina to the outside world.

`=>` `color{blue}"Gestation`: It is the process or period of `color{Violet}"development of the embryo"` inside the womb of the female viviparous animal between `color{Violet}"conception and birth"`. Gestation comes from the `color{Violet}"Latin gestare,"` which means to `color{Violet}"bear, or give birth to"`.

`=>` `color{blue}"Foetal Ejection Reflex"` : Foetal ejection reflex are the `color{Violet}"mild uterine contractions"` , at the time of parturition, generated by the placenta when the foetus is fully developed.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● The average duration of human pregnancy is about `color{Violet}"9 month"` which is called the `color{Violet}"gestation period"`.

● `color{Violet}"Vigorous contraction"` of the uterus at the end of pregnancy causes expulsion/delivery of the `color{Violet}"foetus"`. This process of `color{Violet}"delivery"` of the foetus(childbirth) is called `color{Violet}"parturition"`.

● Parturition is induced by a complex `color{Violet}"neuroendocrine mechanism"`.

● The `color{Violet}"signals"` for parturition originate from the `color{Violet}"fully developed fetus"` and the `color{Violet}"placenta"` which induce mild uterine contractions called `color{Violet}"Foetal ejection reflex"`.

● This triggers release of `color{Violet}"oxytocin"` from the `color{Violet}"maternal pituitary"`.

● `color{Violet}"Oxytocin"` acts on the uterine muscle and causes `color{Violet}"stronger"` `color{Violet}"uterine contractions"`, which in turn stimulates further secretion of oxytocin.

● The `color{Violet}"stimulatory reflex"` between the uterine contraction and oxytocin secretion continues resulting in stronger and `color{Violet}"stronger contractions"`.

● This leads to `color{Violet}"expulsion"` `color{Violet}"of the baby"` out of the uterus through the birth canal – `color{Violet}"parturition"`.

● Soon after the infant is delivered, the `color{Violet}"placenta"` is `color{Violet}"also expelled"` out of the uterus.

`=>` `color{blue}"Lactation"`: Lactation is the `color{Violet}"secretion of milk"` by women and female mammals during the period `color{Violet}"after they give birth"`.

`color{green} ✍️ color{green} mathbf("KEY CONCEPT")`

● The mammary glands of the female undergo `color{Violet}"differentiation during pregnancy"` and starts producing `color{Violet}"milk"` towards the end of pregnancy by the process called `color{Violet}"lactation"`.

● This helps the mother in `color{Violet}"feeding"` the newborn.

● The milk produced during the `color{Violet}"initial few days"` of lactation is called `color{Violet}"colostrum"` which contains several `color{Violet}"antibodies (IgA)"` absolutely essential to develop `color{Violet}"resistance"` for the new-born babies.

● `color{Violet}"Breast-feeding"` during the `color{Violet}"initial period"` of infant growth is `color{Violet}"recommended"` by doctors for bringing up a healthy baby.