In pregnancy at week 4, the embryo is now the size of a poppy seed. During this important phase, the embryo starts to burrow into the lining of the uterus.
The fertilized egg has divided into a ball of cells, forming a blastocyst. This blastocyst has now been implanted into the lining of the uterus and is ready to start the embryo stage of pregnancy.
Some of the cells have started to form the embryo and begin forming the babyâ€™s future organs. Other cells are forming the placenta and amniotic sac, where the embryo will grow.
The amniotic sac is filled with fluid that keeps the baby safe, dispersing any outside forces.
The cells that become placenta also become a yolk sac that will nourish the baby until the placenta is able to carry nutrients to the baby. The yolk sac looks like a little ball on an ultrasound.
An interesting fact about twin pregnancy
Based on when the egg begins dividing, itâ€™s possible to determine what type of twins you will have.
If the egg is divided one to three days after fertilization, each fetus has its own amniotic sac .later on, two separate placentae will develop. These are referred to as diamniotic dichorionic twins.
If the egg is divided four to eight days after fertilization, each fetus has its own amniotic sac, later, they will share one placenta . These are called diamniotic monochorionic twins.
If the two embryos form 8 to 13 days after fertilization (when the amniotic sac has already developed).they will have one placenta and one amniotic sac to share.This is called monoamniotic monochorionic twins .This is the rarest type of identical twin pregnancy and can make the pregnancy higher risk.
Non-identical twins (fraternal twins) always have their own placenta and amniotic sac, so they are diamniotic dichorionic. The placentas can also be attached to different parts of the uterus.