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Conception
Fertilization happens when a sperm meets and penetrates an egg. It's also called conception. At this moment, the genetic makeup is complete, including the sex of the baby. Within about three days after conception, the fertilized egg is dividing very fast into many cells. It passes through the fallopian tube into the uterus, where it attaches to the uterine wall. The placenta, which will nourish the baby, also starts to form.
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Period: to
How Long
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Development at 4 Weeks
At this point the baby is developing the structures that will eventually form his face and neck. The heart and blood vessels continue to develop. And the lungs, stomach, and liver start to develop. A home pregnancy test would show positive.
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Development at 8 weeks
The baby is now a little over half an inch in size. Eyelids and ears are forming, and you can see the tip of the nose. The arms and legs are well formed. The fingers and toes grow longer and more distinct.
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Development at 12 Weeks
The baby measures about 2 inches and starts to make its own movements. You may start to feel the top of your uterus above your pubic bone. Your doctor may hear the baby's heartbeat with special instruments. The sex organs of the baby should start to become clear.
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Development at 16 Weeks
The baby now measures about 4.3 to 4.6 inches and weighs about 3.5 ounces. You should be able to feel the top of your uterus about 3 inches below your belly button. The baby's eyes can blink and the heart and blood vessels are fully formed. The baby's fingers and toes have fingerprints.
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Development at 20 Weeks
The baby weighs about 10 ounces and is a little more than 6 inches long. Your uterus should be at the level of your belly button. The baby can suck a thumb, yawn, stretch, and make faces. Soon -- if you haven't already -- you'll feel your baby move, which is called "quickening."
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Time for an Ultrasound
An ultrasound is usually done for all pregnant women at 20 weeks. During this ultrasound, the doctor will make sure that the placenta is healthy and attached normally and that your baby is growing properly. You can see the baby's heartbeat and movement of its body, arms, and legs on the ultrasound. You can usually find out whether it's a boy or a girl at 20 weeks.
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Development at 24 Weeks
The baby weighs about 1.4 pounds now and responds to sounds by moving or increasing his pulse. You may notice jerking motions if he hiccups. With the inner ear fully developed, the baby may be able to sense being upside down in the womb.
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Development at 28 Weeks
The baby weighs about 2 pounds, 6 ounces, and changes position often at this point in pregnancy. If you had to deliver prematurely now, there is a good chance the baby would survive. Ask your doctor about preterm labor warning signs. Now is the time to register for birthing classes. Birthing classes prepare you for many aspects of childbirth, including labor and delivery and taking care of your newborn.
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Development at 32 Weeks
The baby weighs almost 4 pounds and is moving around often. The baby's skin has fewer wrinkles as a layer of fat starts to form under the skin. Between now and delivery, your baby will gain up to half his birth weight. Ask your doctor how to do a fetal movement chart. Think about breastfeeding. You may notice a yellowish fluid leaking from your breasts. That is colostrum, and it happens to get your breasts ready for making milk.
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Development at 36 Weeks
Babies differ in size, depending on many factors, such as gender, the number of babies being carried, and size of the parents. So your baby's overall rate of growth is as important as the actual size. On average, a baby at this stage is about 18.5 inches and weighs close to 6 pounds. The brain has been developing rapidly. Lungs are nearly fully developed. Your baby is considered at 'term' when he is 37 weeks. http://www.webmd.com/baby/ss/slideshow-fetal-development