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.
How your life's changing:
You may notice some pregnancy-related discomforts already. Many women report sore breasts, fatigue, and frequent urination starting in the early weeks. You may also have nausea, though it's more likely to show up in the coming weeks.
The outside world won't see any sign of the dramatic developments taking place inside you — except that you're turning down that glass of wine with dinner, perhaps. It's important to avoid alcohol throughout your pregnancy since no one knows exactly how much — or how little — alcohol can harm a developing baby.
You'll also want to continue or start an exercise routine. Exercise helps you develop the strength and endurance you'll need to manage the extra weight you'll be carrying. It may help prevent some of the aches and pains of pregnancy, and many women find that it's a great stress-reducer. Exercise can also help you get ready for the physical rigors of labor.
Finally, it's easier to bounce back after you give birth if you've continued some form of exercise throughout pregnancy. Choose a safe, moderately vigorous activity you enjoy. Walking and swimming are fine choices for pregnant women.