The Inside Story: Your Baby in Weeks 1 Through 4

Week 4

Don't be nervous if you experience a little spotting this week. The embryo burrows into your uterine lining as part of the implantation process, and this sometimes causes a small amount of bleeding. That's normal. Your baby's cells continue to multiply, and the placental tissue grows, inhabiting the lining of the uterus and establishing contact with your circulation, which will supply more blood to your growing baby. The placenta also produces estrogen, progesterone, and hCG (human chorionic gonadotropin), which is often referred to as the "pregnancy hormone."

Although your baby is still a microscopic cluster of cells at this point -- you can't even see it on an ultrasound -- three different essential cell layers are already beginning to develop. Their names are probably familiar from biology class: the ectoderm, which will become your baby's nervous system, hair, and skin; the endoderm, which will form your baby's gastrointestinal tract, liver, pancreas, and thyroid; and the mesoderm, which eventually develops into your baby's skeleton, connective tissue, blood system, urogenital system, and muscles.

Originally published in You & Your Baby: Pregnancy.

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