Prenatal ultrasounds are often exciting for expecting parents. They get to see inside the womb and catch a rare glimpse of their baby-to-be. However, these exams usually only happen once or twice during an uncomplicated low-risk pregnancy.
For example, you might have an ultrasound exam around 20 weeks (sometimes called an anatomy scan) but not get an ultrasound at any other time during your second trimester. This leaves much of fetal development—and what your baby looks like each week—a mystery.
However, since fetal development is fairly uniform, you don't have to study your own baby-to-be's pictures to get an idea of what they likely look like. Here's what to know about second trimester ultrasounds week-by-week.
Editor's Note
While this article uses the terms "gender" and "boy" vs. "girl," it's important to note thatgender is a personal identitythat exists on a spectrum, can change over the course of a person's lifetime—and most importantly—is something that a person defines for themselves. Sex, on the other hand, is assigned at birth based on the appearance of a baby's genitalia. While sex assigned at birth often matches a person's gender (called cisgender), sometimes it does not for transgender, intersex, and gender nonbinary people.
Second Trimester Begins: Weeks 13 to 17 of Pregnancy
14 Week Ultrasound
Fetal Size: Length, 3 1/4 to 4 inches (crown to rump); weight, around 1 ounce
Fetal Development Milestones: For fetuses at 14 weeks of development, male or female genitalia are now formed. Your baby's liver and spleen are producing red blood cells. Also, their upper limbs are proportionate to the rest of the body. The 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, a fetal ultrasound picture shows that the fetus's skeletal system is continuing to develop. 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, the bones appear whiter and brighter in an ultrasound image. These cranium pieces do not come together 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: The fetus'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 may be able to 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 (18 to 20 weeks) your health care provider may request an ultrasound to evaluate the fetus size and anatomy. The sonographer will measure the baby-to-be's head circumference or biparietal diameter (BPD).
A lot of 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. The baby's eyes are now developed enough to detect light.
What's more, bones and nerve endings associated with hearing are developing. The baby's soft bones are beginning to harden too.
Boy or Girl: How Genetics Determine Your Baby's Sex
The sonographer can't always determine a baby's sex during an ultrasound exam (and, of course, the exam is centered on the health of the fetus, not the baby's sex organs). But sometimes they can tell whether you're expecting a boy or a girl.
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. You may be able to feel your baby's movements now or 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 girls, the uterus is now formed and the vagin*l canal is also forming. You can feel fetal movements even more now! The 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! The baby is swallowing amniotic fluid to help develop their lungs and digestive system. Bone marrow is beginning to produce red blood cells. The baby moves and wiggles frequently. Looking at their chest, the sonographer can identify the fetal heart and lung tissue.
Feeling Baby Move: Weeks 18 to 21 of Pregnancy
22 Week Ultrasound
Fetal Size: Length, 7 2/3 inches (crown to rump); weight, 12 1/4 ounces
Fetal Development Milestones: At this stage, a baby-to-be 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-to-be 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 for now, they're still relatively thin.
24 Week Ultrasound
Fetal Size: Length, 8 1/2 inches (crown to rump); weight, 11/4 pounds
Fetal Development Milestones: At 24 weeks, crucial development takes place in the fetus's vital organs. They're capable of reacting to noise with a blink-startle response. Their lungs are developing.
25 Week Ultrasound
Fetal Size: Length, 8 3/4 inches (crown to rump); weight, 1 1/2 pounds
Fetal Development Milestones: Your baby-to-be's hearing has developed enough to hear their parent's 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 the head.
Developing Senses: Weeks 22 to 25 of Pregnancy
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.
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-to-be is at the end of the second trimester. The lungs are continuing to develop, and the liver is maturing. The baby's immune system is strengthening.
Now that they have grown so much, it's harder to get a complete profile in baby ultrasound pictures (they just don't fit in the screen anymore!). Even though they have more than tripled their weight since this 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. Consult with your doctor about their recommendations for when you should get an ultrasound during your pregnancy.
Are Ultrasounds Safe for Babies During Pregnancy?
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Prenatal Ultrasounds and Fetal Development
Prenatal ultrasounds are a significant part of the pregnancy journey for expecting parents. These exams provide a rare opportunity to catch a glimpse of the developing baby inside the womb. While ultrasounds are typically performed once or twice during an uncomplicated low-risk pregnancy, they offer valuable insights into the fetal development at different stages.
Second Trimester Ultrasounds Week-by-Week
- 14 Week Ultrasound: At 14 weeks, male or female genitalia are formed, and the baby's liver and spleen are producing red blood cells. The upper limbs are proportionate to the rest of the body, while the lower limbs are slightly shorter.
- 15 Week Ultrasound: The fetal skeletal system continues to develop, and characteristic hair patterns on the scalp are taking shape. The large bones that make up the skull are forming and hardening.
- 16 Week Ultrasound: The fetus's lower limbs are well developed, toenails are forming, and facial muscles are beginning to work.
- 17 Week Ultrasound: Fetal bones become more visible on ultrasound, and some fetal movement may be felt, particularly for those who have been pregnant before.
- 18 Week Ultrasound: At this stage, significant changes are happening in the baby's head, including the development of ears, facial features, and eyes. Bones and nerve endings associated with hearing are also developing.
- 19 Week Ultrasound: Arms and legs are well-developed and gaining muscle, and fetal movements may become noticeable.
- 20 Week Ultrasound: Hair and nails continue to grow, and in girls, the uterus is formed, and the vagin*l canal is also developing. Fetal movements become more prominent.
- 21 Week Ultrasound: The baby is swallowing amniotic fluid to aid in the development of their lungs and digestive system. Bone marrow is beginning to produce red blood cells, and the fetal heart and lung tissue can be identified.
- 22 Week Ultrasound: Rapid development is observed, including the growth of hair and the formation of eyebrows. The baby's sense of taste and smell are also developing.
- 23 Week Ultrasound: The baby is nearing viability, adding fat tissue and gaining weight. Rapid eye movements (REM) during dreaming begin at this stage.
- 24 Week Ultrasound: Crucial development takes place in the fetus's vital organs, and they are capable of reacting to noise with a blink-startle response. Their lungs are also developing.
- 25 Week Ultrasound: The baby's hearing has developed enough to hear their parent's voice, and hair continues to grow on the head.
- 26 Week Ultrasound: Eyelashes are forming, and scalp hair continues to grow.
- 27 Week Ultrasound: The baby's lungs and liver continue to develop, and their immune system is strengthening. This marks the end of the second trimester.
It's important to note that the article also addresses the distinction between gender and sex, emphasizing that gender is a personal identity that exists on a spectrum, while sex is assigned at birth based on the appearance of a baby's genitalia.
Conclusion
Prenatal ultrasounds provide valuable insights into the development of the fetus during the second trimester of pregnancy. These exams offer expecting parents the opportunity to witness the remarkable growth and development of their baby. It's essential to consult with healthcare providers for recommendations on when ultrasounds should be performed during pregnancy, ensuring that they are conducted for medical purposes and by experienced technicians.
