Your little sesame seedling already has a heart with four chambers, which begins pumping blood this week. It won't yet be audibly discernable to you or your doctor, but the movement of it would likely show up on an ultrasound by week six or seven.

By week five, the embryo is securely nestled into your uterine wall and dividing into three cell layers: the ectoderm, the mesoderm, and the endoderm. The top layer of cells, called the ectoderm, is where the hair, skin, tooth enamel, nails, and mammary and sweat glands will develop. The neural tube, which eventually forms the brain and spinal cord, will start to develop too.

The mesoderm, or middle layer, will make the most impressive changes this week. The mesoderm is also responsible for developing muscles, cartilage, bone, placenta, and the umbilical cord. In the weeks to come, the bottom layer (called the endoderm), will eventually develop your baby's lungs, intestines, thyroid, pancreas, and early urinary system.

By now, the level of hCG in your urine should be high enough for a home test to detect (and confirm!) what you probably already expect - you're pregnant!

This week, you may feel... everything!

Happy, anxious, exhausted, depressed, scared, powerful, sexy, frumpy, if you can name it, you can feel it. It's like an amped-up PMS, but amplified. Exaggerated. As your hormones flare, your emotions will generally do the same, reeling you about your day like one-woman soap opera. Mood swings are a normal part of pregnancy and most intense during the first six months.

It's too early for you to have gained much weight; and even though the first month of pregnancy is behind you, you may still not feel pregnant. Furthermore, women who experience a lot of morning sickness may even lose some weight during the first month of pregnancy.

If you're experiencing morning sickness, some moms-to-be find it helps to eat six smaller, high-carb meals (instead of three, large meals). Other moms-to-be advise avoiding fatty foods and keeping saltine crackers in your purse for any spur-of-the-moment queasy attacks. If nausea and vomiting are keeping you from food right now, don't worry. Your little one doesn't need you to add extra calories to your diet yet. During the first trimester, the average weight gain is usually about three to five pounds.