How Big Is a Fetus at 20 Weeks?

Dominique W. Brooks
Woman looking at sonogram
A pregnant woman can have an ultrasound to find out how big her fetus is at 20 weeks.

Asking "how big is a fetus at 20 weeks?" is very common when you reach the midway point of your pregnancy.

Changes in the Mother at 20 Weeks

In order to better understand the changes and growth of your fetus at 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 noticeable belly bump. The top of your uterus is 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. Heartburn, indigestion, and gas become more common as you have less space for your stomach.

How Big Is a Fetus at 20 Weeks?

At 20 weeks, your fetus has undergone a significant number of changes:

  • Your fetus is about 10 inches long from head to toe and around 6 inches from head to rump.
  • The fetus weighs 10.5 ounces.
  • The head circumference at 20 weeks is between 15 to 20 centimeters in size.
  • The humerus -- which is the long bone in the arm -- can range from 2.5 to 3.5 centimeters in length.
  • The femur or the long bone in the leg may be around 2.8 to 3.8 centimeters in length.

These estimates are based on standardized fetal growth charts. 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 80-90 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.

Every Pregnancy Is Different

No two pregnancies are alike and very few fetuses develop in exactly the same way. "How big Is a fetus at 20 weeks?" is a question that can be answered, but you have to remember that all of these measurements are estimates. Most measurements during pregnancy are estimates; you will get accurate readings for height and weight after the baby is born. The doctor needs to be able to hold the baby in hand and use a tape measure or a scale to be accurate.

How Big Is a Fetus at 20 Weeks?