http://www.cnn.com/HEALTH/library/PR/00112.htmlFetal development: What happens during the first trimester?
From MayoClinic.com
Special to CNN.com
To help answer some of these questions, check out this weekly calendar of events for your baby's first three months in the womb.
Week 1: Getting ready
It may seem strange, but you're not actually pregnant the first week or two of the time allotted to your pregnancy. Yes, you read that correctly!
Conception typically occurs about two weeks after your period begins. To calculate your due date, your health care provider will count ahead 40 weeks from the start of your last period. This means your period is counted as part of your pregnancy — even though you weren't pregnant at the time.
Week 2: Fertilization
The sperm and egg unite in the fallopian tube to form a one-celled entity called a zygote. If more than one egg is released and fertilized, you may have multiple zygotes.
The zygote has 46 chromosomes — 23 from you and 23 from your partner. These chromosomes contain genetic material that will determine your baby's sex and traits such as eye color, hair color, height, facial features and — at least to some extent — intelligence and personality.
Soon after fertilization, the zygote will travel down one of your fallopian tubes toward the uterus. At the same time, it will begin dividing rapidly to form a cluster of cells resembling a tiny raspberry. The inner group of cells will become the embryo. The outer group of cells will become the membranes that nourish and protect it.
Week 3: Implantation
The zygote — by this time made up of about 500 cells — is now known as a blastocyst. When it reaches your uterus, the blastocyst will burrow into the uterine wall for nourishment. The placenta, which will nourish your baby throughout the pregnancy, also begins to form.
By the end of this week, you may be celebrating a positive pregnancy test.
Week 4: The embryonic period begins
The fourth week marks the beginning of the embryonic period, when the baby's brain, spinal cord, heart and other organs begin to form. Your baby is now 1/25 of an inch long.
The embryo is now made of three layers. The top layer — the ectoderm — will give rise to a groove along the midline of your baby's body. This will become the neural tube, where your baby's brain, spinal cord, spinal nerves and backbone will develop.
Your baby's heart and a primitive circulatory system will form in the middle layer of cells — the mesoderm. This layer of cells will also serve as the foundation for your baby's bones, muscles, kidneys and much of the reproductive system.
The inner layer of cells — the endoderm — will become a simple tube lined with mucous membranes. Your baby's lungs, intestines and bladder will develop here.
Week 5: Baby's heart begins to beat Your baby at week 5 (three weeks from conception)
At week five, your baby is 1/17 of an inch long — about the size of the tip of a pen.
This week, your baby's heart and circulatory system are taking shape. Your baby's blood vessels will complete a circuit, and his or her heart will begin to beat. Although you won't be able to hear it yet, the motion of your baby's beating heart may be detected with an ultrasound exam.
With these changes, circulation begins — making the circulatory system the first functioning organ system.
Week 6: The neural tube closes
Your baby at week 6 (four weeks from conception)
Growth is rapid this week. Just four weeks after conception, your baby is about 1/8 of an inch long. The neural tube along your baby's back is now closed, and your baby's heart is beating with a regular rhythm.
Basic facial features will begin to appear, including an opening for the mouth and passageways that will make up the inner ear. The digestive and respiratory systems begin to form as well.
Small blocks of tissue that will form your baby's connective tissue, ribs and muscles are developing along your baby's midline. Small buds will soon grow into arms and legs.
Week 7: The umbilical cord appears
Your baby at week 7 (five weeks from conception)
Seven weeks into your pregnancy, your baby is 1/3 of an inch long — a little bigger than the top of a pencil eraser. He or she weighs less than an aspirin tablet.
The umbilical cord — the link between your baby and the placenta — is now clearly visible. The cavities and passages needed to circulate spinal fluid in your baby's brain have formed, but your baby's skull is still transparent.
The arm bud that sprouted last week now resembles a tiny paddle. Your baby's face takes on more definition this week, as a mouth perforation, tiny nostrils and ear indentations become visible.
Week 8: Baby's fingers and toes form
Eight weeks into your pregnancy, your baby is just over 1/2 of an inch long.
Your baby will develop webbed fingers and toes this week. Wrists, elbows and ankles are clearly visible, and your baby's eyelids are beginning to form. The ears, upper lip and tip of the nose also become recognizable.
As your baby's heart becomes more fully developed, it will pump at 150 beats a minute — about twice the usual adult rate.
Week 9: Movement begins
Your baby at week 9 (seven weeks from conception)
Your baby is now nearly 1 inch long and weighs a bit less than 1/8 of an ounce. The embryonic tail at the bottom of your baby's spinal cord is shrinking, helping him or her look less like a tadpole and more like a developing person.
Your baby's head — which is nearly half the size of his or her entire body — is now tucked down onto the chest. Nipples and hair follicles begin to form. Your baby's pancreas, bile ducts, gallbladder and anus are in place. The internal reproductive organs, such as testes or ovaries, start to develop.
Your baby may begin moving this week, but you won't be able to feel for it quite a while yet.
Week 10: Neurons multiply
Your baby at week 10 (eight weeks from conception)
By now, your baby's vital organs have a solid foundation. The embryonic tail has disappeared completely, and your baby has fully separated fingers and toes. The bones of your baby's skeleton begin to form.
This week, your baby's brain will produce almost 250,000 new neurons every minute.
Your baby's eyelids are no longer transparent. The outer ears are starting to assume their final form, and tooth buds are forming as well. If your baby is a boy, his testes will start producing the male hormone testosterone.
Week 11: Baby's sex may be apparent
Your baby at week 11 (nine weeks from conception)
From now until your 20th week of pregnancy — the halfway mark — your baby will increase his or her weight 30 times and will about triple in length. To make sure your baby gets enough nutrients, the blood vessels in your placenta are growing larger and multiplying.
Your baby is now officially described as a fetus. Your baby's ears are moving up and to the side of the head this week. By the end of the week, your baby's external genitalia will develop into a recognizable penis or clitoris and labia majora.
Week 12: Baby's fingernails and toenails appear
Twelve weeks into your pregnancy, your baby is nearly 3 inches long and weighs about 4/5 of an ounce.
This week marks the arrival of fingernails and toenails. Your baby's chin and nose will become more refined as well.
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