Because it's hard to know exactly when conception occurred, it's accepted obstetrical practice to calculate the age of the embryo and fetus from the first day of your last menstrual period. Age calculated this way is called "gestational age," and it’s the way age is calculated during your pregnancy and in this calendar as well. In the first 10 days or so of our calendar, the mother ovulates and her egg starts to travel down the fallopian tube, where it encounters the sperm.
"Fetal age" is the actual age of the fetus from the date of conception. Gestational age is about 2 weeks more than the actual fetal age.

Conception begins when the male reproductive cell, the sperm, successfully fuses with the female reproductive cell, the egg. After the fusion of the sperm and egg, the fusion of their genetic materials follows. The cell then divides. The zygote (the cell formed by the fusion of the sperm and egg) divides into two cells, then four, eight, and so on.

Changes happen in every stage of embryonic development. At 25 days, the embryo curves into a C shape and the arches that form the face and neck are becoming evident under the enlarging forebrain. The primitive heart is beating. The development of the limb buds is also visible, and the hand plates are noticeable in day 44. At 56 days, development of the circulatory system is foremost. Other embryonic organs develop as well: the brain, heart, umbilical cord, vertebrae, stomach, kidneys, lungs, and liver. At this stage, all the major organs are in place.

The genitalia of a 56-day-old fetus inside the womb are visible. Though the indifferent penis can be seen, the sex of the baby is not clear from external appearance until week 12. At 7 months, we can see that the fetus is in fetal position, in which the legs are drawn up due to the limited space in the uterus. The arms, legs, and toenails are fully formed. At 9 months, the skull shows the unconnected bony plates called fontanels. These fontanels are necessary for protection of the fetal brain, as they allow the head to elongate and mold during childbirth and then return to a rounded shape afterward.

At 9 months, the baby is ready for delivery. The baby is in position for birth and then rotates down, pushes out, and comes out headfirst. In order to squeeze the baby through the pelvic canal, the ligaments of the mother's pelvic bones stretch, allowing the pelvis to spread open.