Your pregnancy is dated from the start of your last period, although conception usually occurs 2 weeks later. Thus, by the time you miss a period, you will probably be 5 weeks pregnant.
A full-term pregnancy lasts 40 weeks, so your estimated date of delivery will be 40 weeks from your last menstrual period. Making a note of this date helps the midwife when it comes to working out your due date.
Development of baby till 3 weeks:
Approximately 14 days before your period is due, your body releases an egg and the lining of your uterus starts to thicken. It only takes one sperm-out of the millions that are released and the hundreds that make the long journey up the fallopian tube-to fertilize an egg.
The tail separates from the head of the sperm as soon as it penetrates the egg and cell division begins. Within hours, the fertilized egg becomes two cells, then four, then eight and so on, and is called a zygote. Approximately 4 days after fertilization it is a solid cluster of cells, called morula.
About a week after fertilization, the balls of cells-now hollow in the centre called blast cyst-reaches uterus. The blast-cyst will attach itself to the wall of uterus and begin to embed deep into its lining. This process is called implantation.
The cluster cells very quickly produce an outer layer, which will develop into the placenta and amniotic sac, and an inner layer, which will develop into the embryo. The outer layer has root like structures that bury into the lining of the uterus. These become the route by which nutrients and oxygen are transported from your circulation to what will soon be the developing placenta and embryo.
The morula is microscopic in size and resembles a mulberry. By the time it implants in the uterus, at around day 10, it resembles a grayish blackberry and is called a blast-cyst. This hollow cluster of cells will grow and develop into the embryo.
Development at 4 weeks:
At 4 weeks the cells are dividing and multiplying rapidly, and three layers of cells have now formed. The outer layer will develop into your baby’s brain, nervous system, skin, hair, nails and teeth. The middle layer will become their heart and blood vessels, bones, muscles and reproductive organs. The inner layer will develop into her lungs, liver, bladder and digestive system.
The cells of the embryo grow lengthwise, so that the initially round cluster of cells assumes a leaner shape. The outer cells extend tiny fingers-like projections which link up with your circulation.