The zygote divides into two cells after about 30 hours, four cells after 40 hours, and 12-16 cells in 3 days. As it divides, it’s slowly carried down the fallopian tube by a series of tubal muscle movements. When it reaches the 16-cell stage, it’s called a morula, meaning “little mulberry.” Approximately 72 hours after fertilization, the morula reaches the uterus, where it floats around for about 2 days. Now, for the first time, the morula starts to divide into the specialized cells that will later form fetal structures. A fluid-filled cavity forms inside the morula, transforming it into a hollow ball called a blastocyst.

The blastocyst continues to divide until it has a few hundred cells: only the size of the tip of a ballpoint pen, but big enough to require additional food and oxygen to sustain it. Meanwhile, hormones from your ovaries have been bathing the uterine lining, making it soft and porous. The blastocyst attaches itself to the receptive lining and then, using knifelike enzymes, burrows deep inside it, rupturing capillaries so that it is awash in nourishing blood. The chorion, a tough membrane, forms, and fingerlike extensions reach out of it and tap into your circulation. The growing life inside you has secured a home that will provide it with food and shelter for the next 9 months.