Once your baby is conceived, he or she gets to work right away. The next step in the process is cell division, Within about 12 hours after fertilization, your one-celled zygote divides into two cells and then those two each split into two, and so on, with the number of cells doubling every 12 hours.
Also within about 12 days after fertilization, the placenta begins to form. At first, tiny projections sprout from the wall of the blastocyst. From these sprouts, wavy masses of tiny blood-vessel-filled tissue develop. Called chorionic villi, these masses grow amid the capillaries of your uterus and ultimately cover most of the placenta.
At 14 days after conception, four weeks since your last menstrual period, your baby is about 1/25 of an inch long. It's divided into three different layers, from which all tissues and organs will eventually develop.
Fetal development is still in the very early stages, but many great changes are taking place! The implanted blastocyst is embedded more deeply into the lining of your uterus, and the amniotic cavity, which will fill with amniotic fluid, is starting to forum. Vascular networks that contain maternal blood are becoming established.
Secondary smoke may harm a non-smoking woman and her developing baby. Ask those who smoke to refrain from smoking around you during pregnancy.