Prenatal ultrasounds are a wonderful opportunity to get a glimpse of your baby before he or she is born. However, in low-risk, uncomplicated pregnancies, these tests are usually only done once or twice. You may have the chance to see your baby in one ultrasound (sometimes called an anatomy ultrasound) between 18 and 23 weeks of pregnancy, but then you won’t be able to see your baby again until he or she is born.
But the second trimester is a time of important milestones in fetal development: tooth buds, nails, eyebrows and eyelashes appear, your baby starts moving, stretching and even having a bowel movement. Throughout this process, you may be wondering how your baby is developing. Here, we discuss your baby’s size, characteristics and behavior from weeks 13 to 27.
14 Week Ultrasound
Fetus size: 3 1/4 to 4 inches long (head to butt) and weighs about 1 ounce
Fetal Development Milestones: By the 14th week, the genitals are formed. Your baby’s liver and spleen are making red blood cells. Additionally, your baby’s upper limbs are in proportion to the rest of his or her body. Your baby’s lower limbs are slightly shorter.
15 Week Ultrasound
Fetus size: Length 4-4 1/2 inches (head to buttocks); Weight: 1 3/4 ounces
Fetal Development Milestones: At 15 weeks, an ultrasound can show how the skeletal system is developing. A characteristic hair pattern forms on the scalp.
Your fetus is about the size of a grapefruit. The large bones that make up your baby’s skull are forming and hardening. As the bones harden, they appear whiter and brighter on an ultrasound image. These skull fragments don’t fuse until after birth.
16 Week Ultrasound
Fetus size: 4 1/3 to 4 2/3 inches long (head to butt) Weight: 2 3/4 ounces
Fetal Development Milestones: Your baby’s lower limbs are fully developed. Your baby’s toenails are forming. Your baby’s eyes and ears continue to move into place. Your facial muscles are beginning to move.
17 Week Ultrasound
Fetal size: Length 4 1/2 to 4 3/4 inches (head to buttocks); Weight, 3 1/2 ounces
Fetal Development Milestones: Your baby’s bones are now more clearly visible on an ultrasound, and you may feel your baby move, especially if you’ve been pregnant before.
Ultrasound at 18 weeks
Fetal size: Length, 5-5 1/2 inches (head to buttocks); Weight, 5 1/4 ounces
Fetal development milestones: Around the middle of your pregnancy (about 18 to 20 weeks), your healthcare provider may order an ultrasound to evaluate your baby’s size and structure. The ultrasound technician will measure your baby’s head circumference, or head parietal diameter (BPD).
At 18 weeks, major changes are happening inside your baby’s head. Your baby’s ears are starting to form. His facial features are complete. His eyes are developed enough to detect light. Plus, the bones and nerve endings involved in hearing are developing.
19 Week Ultrasound
Fetal size: 5 1/4 to 6 inches long (head to buttocks); weight: 7 ounces
Fetal Development Milestones: Arms and legs are well developed and proportioned, and muscles are developing. If you haven’t felt your baby move yet, you should within the next 1-2 weeks.
20 Week Ultrasound
Fetal size: 5 2/3 to 6 1/2 inches long (head to buttocks); weight: 9 ounces
Fetal Development Milestones: Hair and nails continue to grow. In female fetuses, the uterus is forming and the vaginal canal is forming. You may even feel more baby movements! Although your baby may look thin on your 20 week pregnant ultrasound, he or she is likely doing well developmentally.
21 week ultrasound
Fetal size: Length 7 1/4 inches (head to buttocks) Weight 10 1/2 ounces
Fetal Development Milestones: At 21 weeks pregnant, you’re in the second half of your pregnancy. Your baby is swallowing amniotic fluid to help develop his lungs and digestive system. Your bone marrow is starting to make red blood cells. Your baby is moving and squirming a lot. Your ultrasound doctor can identify your baby’s heart and lung tissue.
22 Week Ultrasound
Fetal size: Length 7 2/3 inches (head to buttocks) Weight 12 1/4 ounces
Fetal Development Milestones: At this stage, your baby is growing rapidly. Your baby’s hair is growing and eyebrows may be starting to form. Your baby’s sense of taste and smell is also developing.
23 Week Ultrasound
Fetus size: 8 inches long (head to butt) Weight: Approximately 1 pound
Fetal development stages: At 23 weeks, your baby is approaching viability, which means he or she can survive outside the uterus. He or she is gaining weight through the accumulation of fatty tissue. He or she is entering rapid eye movement sleep (REM sleep), the period during which you dream. Your baby will gain a lot of weight over the next few weeks, but will still appear relatively thin on an ultrasound at 23 weeks pregnant.
24th Week Ultrasound
Fetal size: Length 8 1/2 inches (head to butt) Weight, 1 1/4 pounds
Fetal Development Milestones: By 24 weeks, your baby’s vital organs are developing rapidly. Your baby can respond to noises by blinking and reacting with startle.
25th Week Ultrasound
Fetal size: Length, 8 3/4 inches (head to buttocks); Weight, 1 1/2 pounds
Fetal Development Milestones: Your baby’s hearing is developed enough that he or she can hear your voice. Although your baby’s hearing is still immature, they are able to recognize their mother’s voice when they are born because they are constantly listening to her voice while in the womb. Your baby’s hair continues to grow.
Ultrasound at 26 weeks
Fetus size: 9 1/4 inches long (head to butt) Weight: Approximately 2 pounds
Fetal Development Milestones: Your baby’s eyelashes form and the hair on your baby’s scalp continues to grow. Your baby is developing the startle or Moro reflex, the palmar (hand) grasp reflex, and the plantar (foot) grasp reflex. Your baby continues to develop these reflexes after birth.
Ultrasound at 27 weeks
Fetus size: 9 2/3 inches long (head to butt) and approximately 15 1/4 inches long. Weight, 2 lbs.
Fetal Development Milestones: At 27 weeks pregnant, your baby is nearing the end of the second trimester. Your baby’s lungs continue to develop and his liver is maturing. Your baby’s immune system is strengthening.
Baby has gotten so big that it’s becoming harder to get a full picture of him on the ultrasound. He no longer fits on the screen. Baby has tripled in weight since I started my second trimester, but he still has room to grow. This week marks the end of my second trimester, with only one more trimester to go.
Learn more about prenatal ultrasound
Depending on the needs of your pregnancy, ultrasounds may be done during the first and even third trimester, in addition to the second. The timing and number of ultrasounds needed for each pregnancy varies greatly. While it’s natural to want to “see” your baby during these appointments, ultrasounds should only be done for medical purposes and by experienced technicians. Talk to your doctor about when you should have ultrasounds during your pregnancy.
