Fetus Growth and Developmental Milestones

Fetal growth in utero

During the gestational period of pregnancy, there are several different fetus growth stages that occur. Beginning with what looks like a small bean on an ultrasound and going to a fully-developed baby several weeks before the end of a pregnancy, the fetus goes through many different stages. In the last weeks of the pregnancy, the baby is fully developed but puts on a lot of fat while further developing internal systems.

Following Fetus Growth Stages

Many pregnant women like to follow the stages of fetal growth during their pregnancy. One way to do this is to sign up for a weekly email from a website that has pictures and information about every week of pregnancy; in this way, you get the images and information right in your mailbox each week. Sign up for an account at the BabyCenter website in order to receive weekly emails with this fun and valuable information.

Another way to keep track of your own baby's growth stages based on how far along your pregnancy is can be to look up the number of weeks that you are pregnant every so often on the Internet. A few good online sources for learning about fetal development and seeing the development in pictures, are:

Length and Weight in Fetal Growth

The following numbers are averages; all babies grow at different rates and there is a difference between male and female babies, especially nearing the end of pregnancy. When looking at the length figures below, keep in mind that until 20 weeks of pregnancy, the length of your baby is measured from the head to the baby's bottom (because the legs are always curled up and can't really be measured). After 20 weeks, your baby's length is measured from toes to head, hence the sudden jump in height in the figures below:

  • 8 weeks: Average fetal length is .63 inches and weight is 0.04 ounce
  • 12 weeks: 2.13 inches is the average length and average weight is 0.49 ounce
  • 16 weeks: 4.57 inches long and 3.53 ounces
  • 20 weeks: 10.08 inches long (from head to toe) and 10.58 ounces
  • 24 weeks: 11.81 inches and 1.32 pounds
  • 28 weeks: 14.80 inches and 2.22 pounds
  • 32 weeks: 16.69 inches and 3.75 pounds
  • 36 weeks: 18.66 inches and 5.78 pounds
  • 40 weeks: 20.16 inches and 7.63 pounds

As is evident in the numbers above, the fetus grows faster and faster as the pregnancy progresses, with especially a significant weight gain at the end of the pregnancy. Women who go past their due date, which is quite common especially in a first pregnancy, often deliver even larger babies.

Fetal Development

Another important aspect in fetus growth stages is to consider which body parts, internal organs, and systems develop during which parts of a pregnancy. While the gender of the baby is already determined at the moment that conception occurs, the baby's gender will not be visible until a few weeks before your 20-week ultrasound. At this point, your baby's genitals and most other physical traits will be visible (depending on the position of the baby during the ultrasound).

Important Milestones

  • 3 weeks: the heart begins to beat
  • 4 weeks: the backbone is forming while arms and legs are starting to become distinct physical features
  • 6 weeks: the brain can control muscle movements
  • 8 weeks: the fetus is now physically complete
  • 16 weeks: everything going on in the environment of the pregnant woman can be heard by the baby
  • 24 weeks: oil and sweat glands start functioning
  • 29 weeks: the baby blinks in response to light seen through the uterine walls
  • 32 weeks: antibody production increases

Following this point in pregnancy, the fetus is fully developed. The last weeks of pregnancy allow the baby to put on weight and gain strength for the outside world.

Learning More

Due to modern technology, there is a veritable wealth of information available to pregnant women about fetus growth stages. Read more about fetal development and the stages of pregnancy so that you know how both you and your baby are changing during the exciting period that is pregnancy.

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Fetus Growth and Developmental Milestones