Note: The animation counts the weeks of pregnancy. (Beginning date = fertilization of the oocyte). Practitioners often use weeks of amenorrhea (beginning date = first days of the last menstruations). You must add 2 weeks to the pregnancy weeks (PW) to convert them into amenorrhea weeks (AW).
The fertilization of the ovule marks the beginning of the pregnancy. At this stage, the egg is inside the fallopian tube, slowly making its way to the uterus.
The number of cells increases rapidly because of cellular division. The egg is then called "morula" (meaning small berry).
Approximately one week after the fertilization, the egg will attach itself to the lining of the uterus. This is called the implantation. (See animation titled Fertilization).
Weeks 2-3 (PW): The egg is implanted inside the uterus. The hormone levels start to change in a more significant way. The mother does not feel anything at this point but, a pregnancy test can easily show that the mother is pregnant. The cells start to specialize themselves. One part becomes the embryo and the other, the placenta. The placenta's role is to bring oxygen and nutrients to the embryo.
Week 4-5 (PW): The embryo expands and divides into three embryonic layers (Ectoderm, Mesoderm and Endoderm) which are all responsible to give different organs to the future baby. The mother starts to feel the first signs of pregnancy.
The organs are almost all there by the second month but are not yet functional. After the third month, the embryo becomes a "fetus". A unicellular egg needs 9 months to become a complex organism of over 3 billion cells.
AW= Amenorrhea Weeks