Fetal development begins before you even know you're pregnant. Here's a weekly calendar of events for the first trimester of pregnancy.
You're pregnant. Congratulations! You'll undoubtedly spend the months ahead wondering how your baby is growing and developing. What does your baby look like? How big is he or she? When will you hear the heartbeat?
Fetal development typically follows a predictable course. 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 readyI
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 one of your fallopian tubes 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 travels down the fallopian tube 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
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, blood circulation begins — making the circulatory system the first functioning organ system. Illustration of embryo three weeks after conception Your baby at week five (three weeks after conception)
You're pregnant. Congratulations! You'll undoubtedly spend the months ahead wondering how your baby is growing and developing. What does your baby look like? How big is he or she? When will you hear the heartbeat?
Fetal development typically follows a predictable course. 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 readyI
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 one of your fallopian tubes 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 travels down the fallopian tube 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
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, blood circulation begins — making the circulatory system the first functioning organ system. Illustration of embryo three weeks after conception Your baby at week five (three weeks after conception)
Week 6: The neural tube closes
Illustration of embryo four weeks after conception. Your baby at week six (four weeks after 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.
Illustration of embryo four weeks after conception. Your baby at week six (four weeks after 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.
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