Now is the time a woman's menstrual cycle is late and surging hormones begin to create signs and symptoms of pregnancy in the mother's body. These strong hormones prevent further ovulation and help the body change to preserve the developing baby.
The ball of cells we saw previously, is now taking shape and cells are beginning to differentiate. Three distinct types of cells now emerge in three layers, called germ layers. The ectoderm cells will become the nervous system, skin and hair. The endoderm becomes the gastrointestinal tract, liver, pancreas and other abdominal organs. The mesoderm will become the skeletal system, blood and vessels, muscles and other connective tissues.
By the end of the fourth week of pregnancy the umbilical cord, with three distinct blood vessels, will be visible. The baby begins to grow as the spinal cord begins to develop in the early, open neural tube. Tiny buds are visible that will become the arms and legs later and most of the major organs are beginning to form. Even the head is visible and the lobes of the brain have formed.
Already your baby has a tiny, beating heart. While it likely only has two main chambers, this vital organ has already begun working.
Even with all these major organs beginning to form, and major systems in place, the baby is still very small. By the end of the fourth week of pregnancy, the embryo is around 1/2 centimeter long and weighs just a 1/2 gram.
Taking a pregnancy test at this point will likely show a positive pregnancy sign - allowing women the celebration of the growing life in their womb. Congratulations!
Did you enjoy this article? It is part of the Pregnancy - Week by Week Through the First Trimester ebook available today!
Resources:
The Pregnancy Book: Month-by-Month by Dr. Sears
Your Pregnancy Week by Week by Lesley Regan
The Complete Book of Pregnancy and Childbirth by Sheila Kitzinger
