Fetal Development in the First Trimester

Welcome to pregnancy!

The first 12 weeks of your pregnancy is an important time in the development of your baby. Caring for this new life can be both exciting and exhausting!

Here are some things to know about your first trimester:  

Embryo, fetus, trimester…what do these terms mean?

Pregnancy is divided into three parts, called “trimesters.” The first trimester of your pregnancy is about 3 months lasting from week 1 until 12 completed weeks.

During the first 8 weeks, your baby is called an embryo. After 8 weeks, your child is referred to as a fetus, meaning offspring.

The first trimester is very important

Your baby is most vulnerable during the first trimester. All the major organs and body systems are forming.

If you suspect you are pregnant, take care to eliminate, or minimize, over-the-counter medications. Consult your healthcare provider regarding any prescription medications you take. Exposure to alcohol, tobacco, drugs, chemical and toxic substances, radiation, and infectious diseases may be harmful to your baby’s development. 

Although your baby’s organs and body systems are completely formed by the end of 12 weeks, your baby will not be able to survive independently. Your baby will continue growing at a more rapid pace in the second and third trimesters.

First trimester growth and development milestones

Just like a child has developmental milestones, so will your baby. Below you will find the first trimester milestones for most normal pregnancies:

At the end of 4 weeks, your baby’s

  • Heart is beating
  • Neural tube, which will become the brain and spinal cord, is forming
  • Digestive system, heart and circulatory system are forming
  • Eyes and ears are starting to develop
  • Tiny limb buds are growing which will later become arms and legs

At the end of 8 weeks, your baby’s

  • Heartbeat can be heard with ultrasound
  • Major body systems including the circulatory, nervous, digestive, and urinary continue developing
  • Head is larger than the rest of the body, but the embryo is taking on a human shape and is about 1-1.5 inches long
  • Mouth is developing tooth buds that will later grow into baby teeth
  • Eyes, nose, mouth and ears are starting to look more distinct
  • Limb buds are looking more like arms and legs and there are little webbed versions of fingers and toes
  • Bones have started growing
  • Movements can’t be felt yet, but they will soon!

At the end of 12 weeks, your baby’s

  • Arms and legs are formed and fingernails and toenails have appeared
  • External genital organs have developed
  • Voice box is beginning to form
  • Eyelids have formed
  • Movements are increasing, but still aren’t likely to be felt