Source: http://www.fitpregnancy.com/
Week 1
Health care providers start counting how many weeks pregnant you are by the date of the first day of your most recent menstrual period (last menstrual period, or LMP). This is used to calculate your baby�s gestational age. When you visit your caregiver, he will probably use a forty-week medical standard as a timetable for the length of your pregnancy. That means that according to most week calculations, you�re already two weeks pregnant on the day you conceive.
Week 2
Once sperm meets egg, a zygote is formed, which will implant itself into the wall of your uterus, becoming what's called an embryo. About 60 percent of fertilized eggs are not successfully implanted and are instead passed within 12 days of conception.
Week 3
Though it's very early in your pregnancy, things are definitely happening! Once the "winning" sperm (one of 200 million or so contenders) has penetrated your egg, the egg shuts down, admitting no more sperm. Two sets of cell nuclei fuse together inside the egg, assigning your baby—now called a zygote—a gender, eye and hair color and more than 200 other genetically determined characteristics.
Week 4
Did you know that in the first formative weeks your baby resembles the embryo of any mammal, looking much like a tadpole? By week seven, your baby will still have a "tail" but is beginning to form a digestive tract, lungs, nostrils, hands, feet and mouth. An ultrasound is able to detect a beating heart.
Week 5
Your embryo is about two millimeters long, about the size of a grain of sand. Your baby transforms into a bundle of cells organized in a C-shape with a top, bottom, front, and back. A groove has developed on the embryo's back, which will seal and develop into the neural tube (which later will become the spinal cord). At this point, the tube already has a wider, flatter top that will grow into your baby's brain. A bulge has developed in the center of the embryo, which will soon become a tiny U-Shaped tube which will form the heart. Your embryo is encased in protective membranes and attached to a yolk sac, which manufactures the embryo's unique blood cells.
Week 6
Your baby and the yolk sac are about the size of an M&M's candy. A month after conception, your embryo looks something like a newt or a tadpole, and it has gills like a fish! Right now, the embryo of your future baby looks much like the embryo of any other animal—a bird, rabbit, or monkey. It has two tiny cups of pigment on the side of its head that will develop into eyes. Tiny buds that will form the lungs have appeared. The neural tube has closed. On end is flattening and expanding to become the brain, and the other end will become the spine. It's already 10,000 times larger than the fertilized egg. The embryo doesn't have gender characteristics yet, but has little dots where the nipples will be, whether it's a boy or a girl. The heart, a tiny U-shaped tube, will start beating between days twenty-one and twenty-four and is circulating the embryo's own blood. It has a small mouth and lips and fingernails are forming.
Week 7
Your baby enters its second month of development, weighing no more than a chocolate chip or a berry. It's about five to thirteen millimeters long (less than half an inch), and weighs less than a gram (0.8g), or less than one-twentieth of an ounce. The human blueprints are already visible. Your child still has a tail but is also beginning to form a digestive tract, lungs, nostrils, hands and feet, and a bump of a mouth. The liver, tongue, and lenses of your baby's eyes are forming. There are beds for your baby's fingernails, and the buds of teeth are forming in the gums. If you could take a picture, your baby would look more like a baby and less like a reptile. The baby's nerve channels and muscles are connecting, and the body can wiggle when the cells communicate. In just two days, from days thirty-one to thirty-three, the brain becomes one-quarter larger. If you were to have an ultrasound, it would be able to detect the beating heart, which shows up looking like a tiny flashing light.
Week 8
Your baby is now about the size of your thumbprint: one-half to three-quarters of an inch. Six weeks is barely enough time to start a magazine subscription, but it's enough time for your fetus to develop limbs, tiny fingers and toes, the beginnings of external ear structures, eyelids, an upper lip, the tip of a nose, and intestines! The outer cells of the embryo have grown links to your blood supply.
Week 9
Your baby is now about three-quarters of an inch long. As the embryo enters its fiftieth day of existence, it becomes known as a fetus. A membrane lid covers your baby's eyes. Your baby's muscles are beginning to develop, and she can make tiny movements. Your child's limbs are growing, but her arms and hands are forming more quickly than her legs and feet. The hands are actually still known as "hand paddles" and look just like they sound. Ridges have formed on the paddles, which will soon become well-defined fingers. Your baby is developing little dimples where her knees and ankles will go, and her elbows are becoming visible. This week is when sex characteristics begin to assert themselves, and ovaries or testes will soon appear (though an ultrasound won't be able to detect specific sex organs for another two months or so). Your baby's brain waves can now be detected.
Week 10
Your baby is now about an inch long and weighs five grams, or one-sixth of an ounce, roughly the size of a garden beetle. This end of the two-month mark is a landmark date for your baby. It's looking more human all the time. If you could look inside, you'd see a thumb tip-size translucent creature that's unmistakably human. Kidneys, lungs, genitals, and the gastrointestinal tract are all present, though far from fully formed. Your baby's bones begin to form in his limbs, a process called ossification. The floor plan for your baby's structure has been laid down, and the next thirty weeks will be about expanding and developing on this blueprint. If your baby is a boy, his testes are already producing testosterone. A Doppler handheld device can usually detect a fetal heartbeat by this point. Once the heartbeat is detectable, your chances of miscarrying in the first trimester are immediately lower: between five and ten percent.
Week 11
Your baby is about 1 1/2 to 2 1/2 inches long and weighs about a third of an ounce, the size of a peanut. This is a big week for your baby's growth—she'll double in height. At the end of the week, her head and body will be roughly equal in length. This week also starts an active phase for her - she can turn somersaults, roll over, flex her fingers, hiccup, and stretch. You won't be able to feel her movement for another month and a half. She's floating in lots of amniotic fluid. Her limbs are developing from webbed paddles into arms and legs that have well-defined fingers and toes. Fingernails, toenails, and hair follicles are also beginning to form. Your baby's testes or ovaries have developed, though the sex probably won't be visible on a sonogram for at least another month. Intestines have developed at the place where the umbilical cord meets your baby's body. The intestines are now able to make constricting movements, though there won't be anything to digest until later.
Week 12
Your baby's crown-to-rump height is 2 1/2 inches, or about as tall as a squash ball. She may weigh as much as half an ounce. This begins the age when the fetus starts to look really cut in those womb pictures. If you had a womb camera, you'd be able to see your baby's proportions changing, with the growth of the head slowing down to let the rest of the body catch up. Arms, legs, and fingers are also growing out and tapering to look more like a newborn's, and your baby's posture becomes less curled and more upright. Isn't it amazing that every person in the world was once the size of your thumb? And for that matter, for every person alive, some woman went through a pregnancy?
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