4 Week Pregnancy, pregnancy at 4 weeks, Baby development at 4 weeks of pregnancy

In first 4 weeks of pregnancy the gestation sac is approximately 3 mm in diameter and the embryo is just big enough to be seen by the naked eye. A pregnancy of 4 weeks is often noted as the beginning of the embryonic period.

Embryo Development in First four / 4 weeks of pregnancy

Your embryo will develop within a bag of fluid in your womb, called the gestation sac. The sac is made up of membranes that contain the amniotic fluid that the baby grows in. During pregnancy week 4 your baby is focusing on implanting into the uterine lining. The ball of cells making up the embryo grows into three distinct layers. The innermost layer, called the endoderm will become the lungs, liver and intestines. The heart and the circulatory system begin to appear in the middle layer. The middle layer called the mesoderm will become the bones making up the skeleton, muscles and also the heart and blood vessels. The outermost layer, called the ectoderm will form the brain and nervous system as well as the skin, hair and the lenses of the eyes. By now the blastocyst or ball of cells that is your baby will be imbedding deeper and deeper into the lining of the uterus. The amniotic cavity is forming and the placenta is starting to form.

The brain and eyes of baby are already starting to develop by the end of week four.

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