During week 4 you may notice that your period is late, although you may have some spotting from the fertilized egg implanting into your uterus. Your placenta continues hormone production and also transports nutrients to the baby as well as oxygen.
Your baby is about 1 millimeter long, and is the shape of a grain of rice. You can see the locations where the eyes, nose, and mouth will all develop. Little buds are starting to grow out of the body, later to form arms and legs. In the umbilical cord you can visually see 3 distinct blood vessels appear. The cells continue to multiply and group together to make different body structures.
There are 3 different layers that are used in the development of the new baby's body.
1- The Ectoderm: The ectoderm consists of the forming of the nervous system, the brain, skin, and hair.
2- The Endoderm: The endoderm consists of the forming of the gastrointestinal tract, the liver, pancreas, and the thyroid.
3- The Mesoderm: The mesoderm forms the skeleton, connective tissues, the blood system, urogenital system, and most of the muscles.
During this week you should still feel quite normal, but you may feel some mild cramping due to implantation.
Some books that I studied from are:
The Pregnancy Book - Everything you need to know from America's baby experts.
By: William Sears, M.D., and Martha Sears, R.N., and Linda Holt, M.D., F.A.C.O.G.
Your Pregnancy - Week by week
By: Dr. Glade Curtis, OB/GYN and Judith Schuler, M.S.
You & Your BABY Pregnancy - Your ultimate week-by-week pregnancy guide
By: Dr. Laura Riley, OB/GYN