Second Trimester Fetal Development: Week by Week

Your baby is growing fast! Here's what you might see on an ultrasound each week.

Pregnant woman having fetal ultrasound
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Prenatal ultrasounds are exciting opportunities to glimpse your baby before they're born. However, these exams usually only happen once or twice during an uncomplicated low-risk pregnancy. You may get a chance to see your baby at a single 18- to 23-week ultrasound appointment—sometimes called an anatomy scan—and then not again until you give birth.

However, the second trimester is packed with fetal development milestones. Tooth buds, fingernails, eyebrows, and eyelashes appear, and your baby begins to move, stretch, and even have bowel movements! Along the way, you may wonder how your baby is growing. Here, we describe your baby's size, features, and behavior from weeks 13 to 27.

14-Week Ultrasound

Fetal Size: Length, 3 1/4 to 4 inches (crown to rump); weight, around 1 ounce

Fetal Development Milestones: By 14 weeks, genitalia are formed. Your baby's liver and spleen are producing red blood cells. Also, their upper limbs are proportionate to the rest of the body. Their lower limbs are slightly shorter.

15-Week Ultrasound

Fetal Size: Length, 4 to 4 1/2 inches (crown to rump); weight, 1 3/4 ounces

Fetal Development Milestones: At 15 weeks, an ultrasound reveals how the skeletal system is developing. Characteristic hair patterns on the scalp are taking shape.

The fetus is about the size of a grapefruit. The large bones that make up their skull are forming and hardening. As they harden, bones appear whiter and brighter in an ultrasound image. These cranium pieces do not fuse until well after birth.

16-Week Ultrasound

Fetal Size: Length, 4 1/3 to 4 2/3 inches (crown to rump); weight, 2 3/4 ounces

Fetal Development Milestones: Your baby's lower limbs are now well developed. Toenails are forming and your baby's eyes and ears continue to move into the proper place. Facial muscles are beginning to work.

17-Week Ultrasound

Fetal Size: Length, 4 1/2 to 4 3/4 inches (crown to rump); weight, 3 1/2 ounces

Fetal Development Milestones: Fetal bones are becoming more visible on ultrasound. You might feel some fetal movement, particularly if you have been pregnant before.

18-Week Ultrasound

Fetal Size: Length, 5 to 5 1/2 inches (crown to rump); weight, 5 1/4 ounces

Fetal Development Milestones: Halfway through your pregnancy (around 18 to 20 weeks) your health care provider may request an ultrasound to evaluate your baby's size and anatomy. The sonographer will measure their head circumference or biparietal diameter (BPD).

Big changes are happening in your baby's head during the 18th week. Their ears are becoming more shapely. Facial features are now in their proper position. Your baby's eyes are now developed enough to detect light. What's more, bones and nerve endings associated with hearing are developing.

19-Week Ultrasound

Fetal Size: Length, 5 1/4 to 6 inches (crown to rump); weight, 7 ounces

Fetal Development Milestones: Arms and legs are now well-developed and in proportion, and they're gaining some muscle. If you haven't felt your baby move around yet, you should within the next week or two.

20-Week Ultrasound

Fetal Size: Length, 5 2/3 to 6 1/2 inches (crown to rump); weight, 9 ounces

Fetal Development Milestones: Hair and nails are continuing to grow. In a female fetus, the uterus is now formed and the vaginal canal is also taking shape. You can feel your baby move even more now! A fetus may look scrawny in 20-week ultrasound pictures, but developmentally, they're likely right on track.

21-Week Ultrasound

Fetal Size: Length, 7 1/4 inches (crown to rump); weight, 10 1/2 ounces

Fetal Development Milestones: At 21 weeks, your pregnancy is in its second half! Your baby is swallowing amniotic fluid to help develop their lungs and digestive system. Bone marrow is beginning to produce red blood cells. Your baby moves and wiggles frequently. A sonographer can identify your baby's heart and lung tissue.

22-Week Ultrasound

Fetal Size: Length, 7 2/3 inches (crown to rump); weight, 12 1/4 ounces

Fetal Development Milestones: At this stage, your baby is developing rapidly. Their hair may be growing, and eyebrows are beginning to form. Their sense of taste and smell are developing too.

23-Week Ultrasound

Fetal Size: Length, 8 inches (crown to rump); weight, almost 1 pound

Fetal Development Milestones: At 23 weeks, your baby is nearing viability, when they may be able to survive outside of the uterus. They're adding fat tissue and gaining weight. Rapid eye movements (REM) during dreaming are now beginning. Your little one is about to experience a dramatic weight gain in the coming weeks, but during a 23-week ultrasound, they'll still appear relatively thin.

24-Week Ultrasound

Fetal Size: Length, 8 1/2 inches (crown to rump); weight, 1 1/4 pounds

Fetal Development Milestones: At 24 weeks, your baby's vital organs are rapidly developing. Your baby is capable of reacting to noise with a blink-startle response.

25-Week Ultrasound

Fetal Size: Length, 8 3/4 inches (crown to rump); weight, 1 1/2 pounds

Fetal Development Milestones: Your baby's hearing has developed enough to hear your voice. Although their hearing is still rudimentary, they'll be able to recognize your voice by the time they're born after hearing it constantly in utero. Hair is continuing to grow on their head.

26-Week Ultrasound

Fetal Size: Length, 9 1/4 inches (crown to rump); weight, nearly 2 pounds

Fetal Development Milestones: Your baby's eyelashes are forming, and their scalp hair continues to grow. They're developing their startle or Moro reflex, palmer (hand) grasp, and plantar (foot) grasp. As newborns, they'll continue developing these reflexes.

27-Week Ultrasound

Fetal Size: Length, 9 2/3 inches (crown to rump); total length about 15 1/4 inches; weight, 2 pounds

Fetal Development Milestones: At 27 weeks, your baby is at the end of the second trimester. Their lungs are continuing to develop, and their liver is maturing. Your baby's immune system is getting stronger and stronger.

Now that they have grown so much, it's harder to get a complete profile in baby ultrasound pictures—they just don't fit on the screen anymore! Even though they have more than tripled their weight since the second trimester began, there's still plenty of growing left to do. This week marks the end of the second trimester—just one more trimester to go.

More About Prenatal Ultrasounds

In addition to the second trimester, ultrasounds are sometimes done in the first trimester and even the third trimester, depending on the needs of the pregnancy. When and how many ultrasounds each pregnancy requires varies greatly. And while it's understandably thrilling to "see" your baby during these exams, ultrasounds should only be done for medical purposes and be performed by experienced technicians. Ask your doctor for their recommendations for when you should get an ultrasound during your pregnancy.

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Sources
Parents uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  4. Flavor Sensing In Utero and Emerging Discriminative Behaviors in the Human Fetus. Psychological Science. 2022.

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