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First Trimester
In the first 2 weeks, the one-celled Zygote multiples and forms a blastocyst. In the 3rd and 4th week, the brain and spinal cord appear. Heart, muscles, ribs, backbone, digestive tract begin to develop. In the 5th through 8th week, external body structures form. By the 9th through 12th week, the embryo becomes a fetus and rapid increase in size begins. -
Period: to
Prenatal Development
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Second Trimester
At 13 to 24 weeks, the fetus continues to enlarge rapidly and the mother can begin to feel the movements. Most of the brain's neurons are preseny by 24weeks and the eyes become sentitive to light. The fetus reacts to sound. -
Third Trimester
From 25-38 weeks, the fetus has a good chance of survival if born udring this time. The size of this fetus incerases and the lungs mature. In the middle of this period, a layer of fat is added under thes kin. Most fetuses rotate into a n upside-down position in preparation for birth.
