From your first pregnancy symptoms to your initial prenatal appointment, the first six weeks of pregnancy are full of exciting moments. Inside the womb, your baby is going through the most rapid developmental period of his life as he makes the journey from fertilized egg to full-fledged fetus.
While it might seem strange, when your doctor counts weeks of pregnancy, she counts back to your last period, not to the actual date of conception. In practical terms, this means that weeks one and two of your pregnancy actually occur before you conceive. Toward the end of this two week period, one of your ovaries releases an egg -- or in some cases, multiple eggs. The egg enters the fallopian tube, and this is where fertilization occurs at the beginning of week three.
Fertilization and Implantation
When the egg and sperm meet in the fallopian tube during week three of pregnancy, the first week of actual gestation, they fuse to become a zygote, the 46-chromosome cell that will become your baby. The zygote divides rapidly as it continues down the fallopian tube, eventually becoming a ball of cells called a blastocyst as it enters the uterus and burrows into the uterine wall at the start of week four of pregnancy, about six days after fertilization. Once implanted, the outer layers of the blastocyst transform into the placenta and the inner cells become the embryo. While a sensitive pregnancy test can pick up the hormonal changes that indicate pregnancy at this point, many women remain unaware that they are pregnant for a few weeks after implantation.
In weeks four and five, the embryo separates into three layers, which begin to form the organs and tissues of the body. The outer layer, the ectoderm, becomes the nervous system and skin. The middle layer, the mesoderm, becomes the circulatory system, muscular system and bones. The inner layer, the endoderm, becomes the digestive and respiratory organs. The placenta continues to grow and attaches itself to the mother's blood supply via the uterine wall. By the middle of the fifth week of pregnancy, the embryo is about 2 millimeters long and has a neural tube and a rudimentary heart and blood vessels. This is typically the time when most women notice their first missed period and start to suspect a pregnancy.
By week six of pregnancy, the fourth week of actual gestation, the heart has begun to beat and might be spotted on an ultrasound scan. The embryo takes on a tadpole-like appearance and limb buds, which will eventually become the arms and legs, start to form. Facial features also begin to develop during week six, although they won't be easy to distinguish for another week or two.