Baby Developments

Your pregnancy: 5 weeks

How your baby's growing:
Deep in your uterus your embryo is growing at a furious pace. At this point, he's about the size of a sesame seed, and he looks more like a tiny tadpole than a human. He's now made up of three layers — the ectoderm, the mesoderm, and the endoderm — which will later form all of his organs and tissues.The neural tube — from which your baby's brain, spinal cord, nerves, and backbone will sprout — is starting to develop in the top layer, called the ectoderm. This layer will also give rise to his skin, hair, nails, mammary and sweat glands, and tooth enamel.His heart and circulatory system begin to form in the middle layer, or mesoderm. (This week, in fact, his tiny heart begins to divide into chambers and beat and pump blood.) The mesoderm will also form your baby's muscles, cartilage, bone, and subcutaneous (under skin) tissue.The third layer, or endoderm, will house his lungs, intestines, and rudimentary urinary system, as well as his thyroid, liver, and pancreas. In the meantime, the primitive placenta and umbilical cord, which deliver nourishment and oxygen to your baby, are already on the job.

2009-02-8 (6 weeks)
Your pregnancy: 6 weeks

How your baby's growing: This week's major developments: The nose, mouth, and ears that you'll spend so much time kissing in eight months are beginning to take shape. If you could see into your uterus, you'd find an oversize head and dark spots where your baby's eyes and nostrils are starting to form. His emerging ears are marked by small depressions on the sides of the head, and his arms and legs by protruding buds. His heart is beating about 100 to 160 times a minute — almost twice as fast as yours — and blood is beginning to course through his body. His intestines are developing, and the bud of tissue that will give rise to his lungs has appeared. His pituitary gland is forming, as are the rest of his brain, muscles, and bones. Right now, your baby is a quarter of an inch long, about the size of a lentil bean.