Sperm–Egg Union and the Rise of the Blastocyst Sexual intercourse enables fertilization as sperm unites with the female egg to initiate pregnancy. Within days, the dividing embryo becomes a morula, a compact cluster that reaches about sixteen cells. As the sixteen‑cell stage is achieved, development advances into the blastocyst.
hCG Shields the Embryo as Chorionic Villi Secure Maternal Supply The hCG hormone prevents the developing baby from being recognized as a foreign entity in the mother’s body and supports development during the third month of pregnancy. Around the second week of development, chorionic projections extend toward the uterine wall and connect with maternal blood. Through this connection, oxygen and nutrients from the mother reach the embryo, while embryonic cells retain the capacity to differentiate into varied types such as bone and fat.
Placenta Sustains Labor and Must Detach Cleanly During labor, the placenta maintains sufficient oxygen and nutrients so the baby continues to receive them despite temporary reductions in blood flow from uterine contractions. After delivery, the placenta needs to be separated from the baby; if it remains attached, problems can arise. This system sustains the baby until birth and then is safely disengaged.