How Big Is a Fetus at 20 Weeks?

Dominique W. Brooks
Woman looking at sonogram

Asking "how big is a fetus at 20 weeks?" is very common. When you reach the midway point of your pregnancy, it is normal to want to visualize the size of your baby.

Size of Fetus at 20 Weeks

By 20 weeks of pregnancy, your fetus has undergone a number of changes, including significant growth in length and weight. His length is equivalent to a small banana or about the length of the palm of your hand to the tip of your middle finger. By ultrasound measurements:

  • Your fetus is about 10 inches long from the crown of the head to heel and around 6.5 inches from crown to rump.
  • The fetus weighs about 10.5 ounces.
  • The head circumference at 20 weeks is between 15 to 20 centimeters in size.
  • The humerus -- the long bone in the upper arm -- can range from 2.5 to 3.5 centimeters in length.
  • The femur -- the long bone in the upper leg -- measures around 2.8 to 3.8 centimeters in length.

These estimates are based on standardized charts, such as the World Health Organization's fetal growth charts. The measurements of your fetus may be slightly smaller or larger.

Other Characteristics of a 20 Week Fetus

There are other developments occurring in your fetus at 20 weeks as well.

Development of the Fetus

Your fetus is practicing swallowing, which is a great workout for the gastrointestinal system. The intestines are beginning to work and are creating meconium, which is a black sticky waste product. You will see this in the first diaper change or two after birth. The fetus is also practicing breathing.

The fetus's skin is now developing into the multiple layers that will cover the entire body after birth.

Determining the Sex

For women who are not high-risk patients, week 20 is usually when the big ultrasound occurs. The obstetrician and radiology technologist will use the ultrasound probe to get a full view of your baby, measuring the length and weight of the fetus. The clinicians may also look at the baby's heart, kidneys, stomach, intestines, brain, and spinal cord to make sure there are no obvious defects.

At 20 weeks, you make be able to find out the sex of your fetus. If the baby cooperates, the technologist should be able to see the three lines that represent the labia folds for girls or a penis and scrotum for boys. If the baby allows a look during the ultrasound, the prediction of the sex is very accurate--around 80 to 90 percent. If you don't want to know the sex, let the doctor and technician know beforehand so you can avoid any unwanted peeks.

While ultrasounds take around half an hour and are painless, they are not necessary procedures during a healthy pregnancy. Many moms-to-be ask for one ultrasound during pregnancy for the determination of the sex and also for reassurance that the fetus is developing normally. Most obstetricians perform at least one ultrasound routinely during a normal pregnancy.

Changes in the Mother at 20 Weeks

In addition to the growth in size and other changes in your fetus by 20 weeks, it is important to look at the changes in your own body at this point. At this stage of your pregnancy, energy is generally not a problem; you may even notice an increase in your sex drive during the second trimester of your pregnancy.

You have started to "show" at this point and have a more noticeable belly bump. With the growth of your baby, the top of your uterus is now level with your belly button. The height of your uterus is called the fundal height, which your obstetrician measures during each visit using a tape measure.

Physical discomforts may begin to accumulate for you at 20 weeks. You may start to notice more pain in your back as your belly grows and your ankles and fingers may begin to swell. In addition, heartburn, indigestion, and gas become more common as you have less space for your stomach.

Every Pregnancy Is Different

No two pregnancies are alike, and very few fetuses develop in exactly the same way. You also have to remember that all measurements of your baby during your pregnancy are estimates. Your doctor will get more accurate readings for height and weight with a tape measure and a scale after your baby is born.

How Big Is a Fetus at 20 Weeks?