Your Baby's Development

Pregnancy is generally divided into trimesters. Your first trimester of pregnancy ends at about 12 weeks, or three months after your last menstrual period. Your doctor may discuss your progress in weeks, which are measured from the first day of your last menstrual period — the day your doctor uses to calculate your due date and the baby's gestational age.

Since it is usually impossible to pinpoint the exact date of ovulation and the date of conception, medical experts use your last menstrual period as the starting point for your next nine months. Basically, this means that the first week of your pregnancy is actually the week that you started your last period.

Therefore, your baby can be up to two weeks younger than his gestational age. Every baby develops differently and at different rates in utero.

The First Month (1 to 4 Weeks)

About two weeks after the first day of your last menstrual period, your ovary released an egg into the fallopian tube. Your actual pregnancy began when that egg was fertilized by a sperm cell. In other words, you are not actually pregnant for the entire first month of pregnancy — weeks one through four.

Over the next week, the fertilized egg grows into a group of cells called a blastocyst. Once the blastocyst completes its journey down the fallopian tube, it implants in the uterus and divides into two parts. One half of the blastocyst attaches to the wall of the uterus and becomes the placenta while the other half develops into the embryo.

This group of cells is already composed of different layers. The outer layer eventually becomes the nervous system, skin, and hair. The middle layer becomes bones, cartilage, muscles, circulatory system, kidneys, and sex organs. The inner layer becomes the respiratory and digestive organs.

The implantation of the egg into the uterus triggers the beginning of hormonal and physical changes. The amniotic sac, which cushions the fetus in the months ahead, begins to form. The early stages of the placenta and umbilical cord are visible and under rapid construction.

During the first month of pregnancy, the embryo looks like a tadpole. The neural tube, which will become the brain and spinal cord, starts to come together. A very primitive face begins to form, with large dark circles where the eyes will be. The mouth, lower jaw, and throat also begin to develop. The baby's blood cells are taking shape, and circulation will soon begin. By the end of the first month, the embryo is about a quarter of an inch long and is smaller than a grain of rice.

The placenta is the interface that provides all the nutrients the baby needs, including oxygen, and takes care of waste disposal. It also produces the hormones progesterone and estriol, which are produced to help maintain a healthy pregnancy. The placenta develops in the uterus just twelve days after conception.

The Second Month (5 to 8 Weeks)

You may not look pregnant yet, but by the second month of your pregnancy, plenty is going on. Major body organs are beginning to develop, including the heart, brain, kidneys, liver, intestines, appendix, lungs, and body systems.

The baby's facial features continue to develop. The baby's ears, fingers, toes, and eyes begin to form. Tiny buds that will become the baby's arms and legs are forming. The digestive tract and sensory organs are now beginning to develop.

During this time, bone starts to replace cartilage. The baby's heart starts its contractions, which will become distinct heartbeats within the next week. The eyelids form and grow — though sealed shut — and nostrils begin to form.

The neural tube will eventually connect the brain and spinal cord, and by about the fifth week it closes. Blood circulation becomes evident at this time. The placenta and amniotic sac continue to develop. By the end of the second month, the embryo has started to look more like a person than a tadpole. It measures about 1 inch long and weighs less than ⅓ ounce.

The Third Month (9 to 12 Weeks)

During your third month of pregnancy, the embryo has developed into a fetus. The baby is active, even though you may not yet be able to feel the activity. All major organs, muscles, and nerves are formed.

The mouth has 20 buds that will eventually become teeth. The irises of the eyes are now forming. The liver, intestines, brain, and lungs are now beginning to function on their own.

At around week eleven, it is possible to hear the “swooshing” sound of the baby's heartbeat for the first time with a special instrument called a Doppler sound-wave stethoscope.

A Doppler stethoscope uses ultrasound to listen to the heartbeat of the fetus. The device is sometimes called a Doptone. The Doppler may be routinely used during your prenatal visits.

Several of the baby's ribs are now visible, and tissue that will eventually form bones is developing around the baby's head, arms, and legs. By the end of your first trimester, or third month, your baby is fully formed. Your little one has arms, hands, fingers, feet, and toes. Fingers and toes are separate, and they now have soft nails. Your baby's reproductive organs are developing, and the circulatory and urinary systems are working. The liver is producing bile.

Throughout the remainder of your pregnancy, the baby's body organs will mature and the fetus will gain weight, become longer, and fully develop. By the end of your third month, your baby is about four inches in length and weighs about 1 ounce. The most critical point of formation of the organs is finished, and your chance of miscarriage at this point drops considerably.

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