Pregnancy dating day 12 of cycle
The heart and major blood vessels begin to develop earlier—by about day 16.The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day.
Most malformations (birth defects) occur during the period when organs are forming.The amniotic sac fills with a clear liquid (amniotic fluid) and expands to envelop the developing embryo, which floats within it.The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side.After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg.The fertilized egg (zygote) divides repeatedly as it moves down the fallopian tube to the uterus. Then it becomes a hollow ball of cells called a blastocyst.Other cells develop into an inner layer of membranes (amnion), which form the amniotic sac.
When the sac is formed (by about day 10 to 12), the blastocyst is considered an embryo.
The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick.
The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta.
At 8 weeks of pregnancy, the placenta and fetus have been developing for 6 weeks.
The placenta forms tiny hairlike projections (villi) that extend into the wall of the uterus.
The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube. In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst.