Fetal development: The 1st trimester

Fetal development begins soon after conception. Find out how your baby grows and develops during the first trimester.

By Mayo Clinic Staff

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 feel the first kick?

Fetal development typically follows a predictable course. Find out what happens during the first trimester by checking out this weekly calendar of events. Keep in mind that measurements are approximate.

Weeks 1 and 2: Getting ready

It might 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 last 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 3: 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 or if the fertilized egg splits into two, you might have multiple zygotes.

The zygote typically has 46 chromosomes — 23 from the biological mother and 23 from the biological father. These chromosomes help determine your baby's sex and physical traits.

Soon after fertilization, the zygote travels down the fallopian tube toward the uterus. At the same time, it will begin dividing to form a cluster of cells resembling a tiny raspberry — a morula.

Week 4: Implantation

The rapidly dividing ball of cells — now known as a blastocyst — has begun to burrow into the uterine lining (endometrium). This process is called implantation.

Within the blastocyst, the inner group of cells will become the embryo. The outer layer will give rise to part of the placenta, which will nourish your baby throughout the pregnancy.

Week 5: Hormone levels increase

The fifth week of pregnancy, or the third week after conception, the levels of HCG hormone produced by the blastocyst quickly increase. This signals your ovaries to stop releasing eggs and produce more estrogen and progesterone. Increased levels of these hormones stop your menstrual period, often the first sign of pregnancy, and fuel the growth of the placenta.

The embryo is now made of three layers. The top layer — the ectoderm — will give rise to your baby's outermost layer of skin, central and peripheral nervous systems, eyes, and inner ears.

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, ligaments, kidneys and much of the reproductive system.

The inner layer of cells — the endoderm — is where your baby's lungs and intestines will develop.

Week 6: The neural tube closes

Growth is rapid this week. Just four weeks after conception, the neural tube along your baby's back is closing. The baby's brain and spinal cord will develop from the neural tube. The heart and other organs also are starting to form.

Structures necessary to the development of the eyes and ears develop. Small buds appear that will soon become arms. Your baby's body begins to take on a C-shaped curvature.

July 12, 2017 See more In-depth