How Big Is a Fetus at 20 Weeks?

How big is a fetus at 20 weeks? Typically, the size of a fetus at 20 weeks will measure about 6.5 inches in length. If using weight to measure the size of a fetus at week 20, it will weigh approximately 10.5 to 11 ounces. This is an estimate, of course, of how big a fetus is at 20 weeks. All fetuses will develop differently.

Interestingly, the fact that we know a fetus at 20 weeks is about 6.5 inches in length says something important about fetal development. The 20-week milestone and the 6.5 inch measurement is significant because, in general, week twenty is when the fetus begins to be officially measured from toes to the top of the head; in earlier months, it is in a curled position and thus is measured from the head to the rump.

Asking how big a fetus is at 20 weeks is also significant because 20 weeks is an important time period in the development of a fetus.

Understanding Fetal Development

Fetal development is an incredible and complex process. When you consider the small clump of cells which are the beginnings of a fetus, compared to the full size of a newborn baby, you can see how quickly and dramatically change and growth takes place in the fetus over the total nine-month period of gestation. At twenty weeks, which is approximately the halfway point of a typical pregnancy, the fetus is quite far along in development.

Rapid Growth of the Fetus

The first twenty weeks are considered the period of most rapid growth, meaning that by week twenty the fetus will be closer to birth size than you may anticipate.

Stage of Development at 20 Weeks

• At twenty weeks, the heart of the fetus also begins to grow stronger, as do the limbs.
• Kicking and movement tends to increase as muscle development moves forward, and the strength of the muscles improves.
• The fetus is also beginning to develop the nerves required for sensory activity like tasting, smelling, touching, seeing, and hearing.  In fact, around this time the fetus can often hear sounds from inside the womb; you may notice increased activity in response to sounds around this time.
• The skeleton of the fetus is also beginning to harden; the bones spend the first several months in the form of rubbery cartilage, which begins to disappear around this time as bone takes its place.
• A fetus at twenty weeks may be just beginning to develop the ability to swallow, and the digestive system, as well as the reproductive system if the fetus is female, is developing.
• The fetus is now producing meconium, which is a black tarry substance that is a byproduct of the practicing of digestion.
• It is also developing the waxy coating that will protect the skin through the rest of the development and birthing process; fat has already been developing on the body for the past four weeks or so, providing even more protection.

At twenty weeks, the fetus is essentially halfway through development, and while actual changes in size will slow down at this time, other changes will begin to happen very rapidly as the date of birth grows closer.