Kunal Sood, MD on Instagram: "Superfetation is the extremely rare situation where a second pregnancy begins during an ongoing one. Normally, pregnancy blocks this through three layers of protection: ovulation stops, cervical mucus thickens into a plug that prevents sperm from entering, and the uterine lining becomes unreceptive to a new embryo. For another conception to occur, all three of these safeguards would need to be bypassed. Documented human cases are very few, and most involve assisted

Embryology: from Fertilization to Gastrulation, Animation