About three days after conception the embryo now has 16 cells and is called a morula and has normally reached the junction of the fallopian tube and the uterus. A couple of days later a cavity develops in the center of the morula. Now this group of cells is called a blastocyst. It has an inner group of cells which will become the fetus and later the newborn baby.
Ten days after conception the embryo has fully attached itself to the endometrium and the placental blood circulation has begun. About the 12 day mark the embryo has started to produce hormones which can be detected in the woman’s urine. This is when the home pregnancy test can usually pick up a positive pregnancy read.
Two weeks, or just 14 days after conception a “primitive streak”, a faint white trace of the caudal, or tail end of the embryonic disk, formed by the movement of cells, providing the first evidence of the embryonic axis. This later develops into the fetus’ central nervous system. One week later, the embryo is now about 1/12 of an inch and at this point the heart begins to beat. Amazing at just 3 weeks! At the 4 week stage the embryo now looks like a small tadpole and has grown to 1/5 of an inch. By the time the embryo is 5 weeks it has developed tiny buds thus being the arms and the legs. A lot has developed in this short amount of time and at this point with an ultrasound you are able to see a small sack with a peanut shaped embryo.
By the 6 week mark the embryo has grown to about a half an inch and has developed eyes. At 8 weeks the primitive or “reptilian brain” begins to form and able to feel pain. A heartbeat can be seen and or heard through an ultrasound. It’s at this stage, anywhere between 8 to 12 weeks after conception is when the embryo is considered a fetus. So I guess there’s no wrong or right answer of when an embryo becomes a fetus, it’s a matter of opinion. Is it when the embryo develops a beating heart, or when the brain can feel pain or when you can actually hear the baby’s heart beating? You decide.
Author: Unique Ultrasound