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How does my baby develop in the first trimester?

Diana Meeks
Diana Meeks on behalf of Sigma Nursing
Family Practitioner

The 1st trimester of pregnancy usually encompasses week 3 of pregnancy to week 13. During this time, there are many changes and developments in your womb! Once your egg is fertilized, it implants in the uterus where it will be nourished and grow. Slowly but surely, your child's cells divide and multiply and begin to form vital organs and structures. This development starts with the brain, spinal cord, and heart. At week 8, your child will begin to move around, though you may not feel it as of yet. At 13 weeks, your child will have changed so much since conception and can be recognized on an ultrasound!

Dr. Mehmet Oz, MD
Cardiologist (Heart Specialist)

The first trimester of pregnancy is a miraculous time in which a single cell develops into an embryo and then a fetus. Learn more about this topic in this animation narrated by Dr. Oz.

Beginning at 3 weeks, the neural tube develops. This becomes the baby’s brain and spinal cord. At about six weeks after conception, your baby was about an inch long, and her brain, nervous system, heart, lungs—and even tiny buds for arms and legs—were all beginning to form. By the end of the first trimester, she will be about 6 inches long and weigh about 4 ounces. Her internal organs will be developing, and her fingers and toes will be completely formed. Your baby is moving around but is still too tiny for the movements to be felt.

The following developments take place during the first trimester (Week 1 through Week 12):

At 4 weeks:

  • Your baby's brain and spinal cord have begun to form.
  • The heart begins to form.
  • Arm and leg buds appear.
  • Your baby is now an embryo and 1/25 of an inch long.

At 8 weeks:

  • All major organs and external body structures have begun to form. The arms and legs grow longer, and fingers and toes have begun to form.
  • Your baby's heart beats with a regular rhythm.
  • The sex organs begin to form.
  • The eyes have moved forward on the face and eyelids have formed.
  • The umbilical cord is clearly visible.
  • At the end of 8 weeks, your baby is a fetus and looks more like a human. Your baby is nearly 1 inch long and weighs less than 1/8 of an ounce.

At 12 weeks:

  • The nerves and muscles begin to work together. Your baby can make a fist.
  • The external sex organs show if your baby is a boy or girl. A woman who has an ultrasound in the second trimester or later might be able to find out the baby's sex.
  • Eyelids close to protect the developing eyes. They will not open again until the 28th week.
  • Head growth has slowed, and your baby is much longer. Now, at about 3 inches long, your baby weighs almost an ounce.

This answer is based on source information from the U.S. Department of Health and Human Services.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.