Once a sperm has entered the egg, changes in the electric charge of the eggs membrane prevent other sperm from entering the egg. At this point, the nucleus of the successful sperm joins with the nucleus of the egg. This joining of nuclei creates a fertilized cell called the zygote. It begins to undergo many cell divisions. After the zygote attaches to the wall of the uterus, it is known as an embryo, it receives nutrients from fluids in the uterus until the placenta develops from tissues of the uterus and the embryo. An umbilical cord develops that connects the embryo to the placenta. I the placenta, materials diffuse between the mothers and the embryos blood, but their bloods do not mix. Blood vessels in the umbilical cord carry nutrients and oxygen from the mothers blood threw the placenta to the embryo.
a zygote containing eight identical cells
a zygote containing eight identical cells

Pregnancy in humans lasts about 38 to 39 weeks during the 3rd week. A thin membrane called the amniotic sac begins to form around the embryo. During the first two months of development, the embryos major organs form and the heart structure begins to beat. At five weeks, the embryo has a head with eyes, nose, and mouth features. During the sixth and seventh weeks, fingers and toes develop.
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After the first two months pf pregnancy, the developing embryo is called a fetus. At this time, body organs are present. Around the third month, the fetus is about 8cm to 9cm long. The mother may feel the fetus move. The fetus may suck its thumb.

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