What to Expect during Fetal Development in the First Trimester

During the first trimester of pregnancy, your baby has taken many important steps in its development toward becoming a baby. In fact, by the end of the first trimester, your baby will have developed its entire body except for the hands and feet, which will come later in the second trimester. To help you understand what happens during fetal development in the first trimester and what this means for you and your growing baby, keep reading to learn more about fetal development during the first trimester from conception through week 12.

Week 1: How Embryos Grow

In week one of fetal development, a mass of cells starts dividing into smaller clusters, each with its own specialized function. Embryos are growing, but they’re still too small and underdeveloped to see anything—even on ultrasound. 

As your embryo grows, it'll start forming three distinct layers of tissue: germ layers (endoderm, ectoderm and mesoderm), circulatory system and central nervous system.

 In weeks two through four of fetal development, your body will develop some organ systems for both you and your fetus. Your organs include skin, bones and skeletal muscles, among others. However, many won't be fully functional until later in pregnancy or after birth. Some changes will occur quickly; others slowly over time.

Week 2: The Embryo starts looking human

At two weeks, we get a glimpse of a blobby embryo that starts looking like a human. We don’t have arms or legs yet, but we do have some recognizable facial features. And those all-important gill slits are just beginning to disappear.

 But don’t worry — though it won’t be much longer until you start seeing your baby as more than a cluster of cells, you still have about eight weeks left before that heart beat kicks up again!

Week 3: The Placenta Forms

The placenta, which connects your baby's blood supply to yours and provides oxygen and nutrients, begins developing. A sac forms around your baby as well, forming a protective coating called amniotic fluid. In Week 3, your baby is called an embryo. It is about 0.08 inches (2 mm) long and weighs about 0.02 ounces (0.6 g).

Week 4: Fingers, toes and eyes start forming

At around 4 weeks, you can already make out some physical characteristics of your baby. Your little one is just a quarter of an inch long, but his or her vital organs are developing. 

The cells that will develop into fingers and toes are forming on your baby's hands and feet; eyes are starting to form on his or her face. By now, all of your baby's bones have begun to harden into cartilage. Most important: Your baby has a heartbeat!

Week 5: The Embryo moves

Although your baby’s movements are often undetectable, by week 5 or 6, you should be able to feel a slight fluttering sensation. However, don’t worry if you don’t feel anything yet; because it is so small, you may have trouble detecting movement at first. 

For some people, it can take a few weeks before they start feeling fetal kicks and punches – but most can feel them around week 10-12.

Week 6: The Muscles are Growing

Although you might not be able to tell, your baby’s muscles are growing. By now, all of her major organs have formed, including her heart and brain. 

However, don’t expect your baby’s brain to resemble yours at birth—it only grows about 1% each week during pregnancy. Your baby still doesn’t have much hair (but he does have all of his), but his eyes and ears are nearly finished developing. 

His tiny fingers and toes continue getting longer as well, now stretching to about a quarter-inch long! Keep an eye on his kidney development throughout first trimester—these will be fully functioning by week 16!

Week 7–8 - Now We See Baby Movements!

You'll probably notice that it's a lot easier for you to feel your baby move now. As a result, you might enjoy feeling more kicks and jabs as early as 14 weeks pregnant.

 Some women even report feeling their babies moving as early as 12 weeks! At 17–20 weeks pregnant, some women experience Braxton Hicks contractions, which may feel like strong cramps or menstrual pain. These aren't cause for concern—they are just practice contractions that help prepare your body for childbirth. Your baby is very busy developing internally at 17–20 weeks of pregnancy.

Week 9–10 - Rapid Growth & Changes to All Systems

The head is nearly half an inch (1.3 cm) long and resembles a tadpole. The beginnings of arms, legs, fingers, and toes become visible as buds on either side of a bulging abdomen. 

Hair can now be seen on your baby’s head, eyebrows and eyelashes are forming. Your baby’s heart is now beating about 100 times per minute! It may also be growing faster than ever before: by one week into pregnancy it had grown from just over one inch (2.5 cm) to almost 3 inches (8 cm).

Notable Events after 12 Weeks

Your baby's organs have begun developing, but are still small enough that they aren't easily seen on an ultrasound. This can make it difficult for doctors to properly gauge fetal age. 

For example, your doctor may estimate that your baby is 12 weeks old when she really is 10 weeks old, which would put her at greater risk of having a chromosomal disorder like Down syndrome or another abnormality. Be sure to tell your doctor how far along you are so that she can get a more accurate picture of fetal development.

Week 11–12 - Fingerprints, Eyesight, Sexual Characteristics & Movement

During week 11, your baby’s external ear begins to develop. Fingerprints have already appeared on his or her tiny fingertips, but they won’t become visible until later on. Your baby’s eyes are also beginning to form. In fact, some babies open their eyes in utero as early as 16 weeks gestation!