Fertilization is the joining of one sperm and one egg to form a single cell with a full set of chromosomes. About a week later, that cell has become a ball of cells that burrows into the uterine lining.
Fertilization
The egg survives about 12 to 24 hours after ovulation, while sperm can survive several days. Fertilization usually happens in the wide outer part of the uterine tube.
Cleavage and travel
The fertilized egg, the zygote, divides again and again as the tube moves it toward the uterus over 3 to 4 days. By the time it arrives it is a hollow ball called a blastocyst.
Implantation
About 6 to 10 days after fertilization, the blastocyst attaches to the endometrium and sinks into it. Its outer cells start to form the placenta and to make hCG.
hCG and the corpus luteum
hCG keeps the corpus luteum making progesterone, so the lining does not shed. hCG is the hormone that pregnancy tests detect in urine and blood.
At the bedside
A urine pregnancy test usually turns positive around the time of the missed period. If the blastocyst implants outside the uterus, most often in the tube, it causes an ectopic pregnancy that can rupture and bleed.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).