It is 2 weeks since the last period started (or the beginning of week 3). It is around this time that your baby is conceived and a new life begins! The woman releases her mature egg (called ovulating) and if sperm are present in her fallopian tubes, there is a 20 to 25% chance her egg will be successfully fertilised to create a baby.
Image 65-03 shows a microscopic image of a ripened egg after being released from the ovary.
Once released, the egg becomes surrounded by thousands of sperm (called spermatozoa). Several stronger sperm burrow their heads into the egg's thick, outer covering. When the most determined sperm reaches the inner part of the egg first, a chemical reaction takes place, which automatically changes the egg's covering to make it impenetrable to other sperm. At the same time the thrashing tail of the successful sperm detaches, leaving only its head within the egg. Combined they are now called the 'conceptus'.
Image 66-07 shows the woman's egg with the head of the successful sperm inside the egg's covering.
Within hours, the conceptus splits into 2 identical cells, then these 2 cells split again into 4 cells. This process continues so that by the 4th day after fertilisation, the conceptus becomes a group of about 16 to 20 cells, called a morula (or 'ball of cells').
Image 66-23 shows the 'morula' as a ball of cells.
On the 5th day after fertilisation, most of the cells in the morula start to move to one side, creating a bunched up group of cells on one side and a sac of fluid (that the baby will float in). These individual cells (which all previously looked the same) now start to change (or differentiate) as they begin preparing to perform their individual functions. Each one destined to become a different part of the baby's body (and eventually the placenta). As cell differentiation continues, the morula becomes what is known as a blastocyst, by day 6 after conception.
While the conceptus is growing into a blastocyst it is also being gently moved down the woman's fallopian tube towards her uterus. Once it enters her uterus it is able to implant in its thick lining (about 1 week before the woman's next period would have been due).
Image 65-07 shows the egg being released from the ovary and being fertilised by the sperm in the fallopian tube. The conceptus changes into a morula and then a blastocyst, before implanting in the lining of the woman's uterus.