LIVESTRONG.com Health: Baby Growth Stages in the Womb

Pregnancy is a rapid growth process inside the womb. Not only is your own body working hard to support your growing pregnancy, but your baby is also growing from two cells to a full-term baby in an average of 40 weeks. Fetal development is typically measured in trimesters, making it possible to track the development inside the womb and determine if your pregnancy is on schedule.

First Trimester

The first trimester is counted from the date of your last menstrual period, as most couples don’t know the exact date of conception, and ends at the 12th week of pregnancy. This means that during weeks one and two, you probably weren’t really pregnant. Most women don’t know they are pregnant until several weeks have gone by, which is why its critical to modify your diet before conception. The cornerstones for fetal development are laid in the first trimester so avoiding alcohol, too much caffeine, raw fish and fish with large amounts of mercury is important to your baby’s future health.

Your baby’s gender, hair color, eyes and basic personality traits are defined at conception, based on the traits passed on by the parents. Your baby also begins to develop her heart, intestines, earlobes, mouth and nose as well as her head, arms, neck and legs in this trimester. Neural tube development begins in the fifth week of pregnancy, which is just about the time you realize your period is late. Neural tubes form the foundation for the brain and spine, and without enough folic acid in your diet you can limit this critical stage. Use prenatal supplements as directed by your obstetrician to avoid any potential development issues.

Second Trimester

The second trimester dates from the 13th to the 27th week of pregnancy. The baby grows from about2 1/2 inches to about 9 inches in this trimester. Your baby’s muscles and organs begin to develop, urine production begins and the genitalia mature, which is why you can find out your baby’s sex during this trimester. Your baby’s bones are forming and hardening at this time. Coupled with his rapid growth, you’re likely to begin to feel him move in the womb in the second trimester. The ears develop and the nerves that make hearing possible form, so that your baby begins to hear your heartbeat and your voice. Your baby develops a swallowing reflex–a critical part of his development–in this trimester as well. Fingernails, hair and fingerprints develop at the end of the trimester. By the 26th week in a normal pregnancy, your baby’s lungs produce surfactant, a substance that prevents the lungs from sticking together. This is a crucial substance if the baby is born early.

Third Trimester

The third trimester dates from the 28th week of pregnancy until birth. During this time, the baby’s eyes open, her bones continue to develop and, most importantly, she begins to practice using her lungs to breathe. She will also rapidly put on weight. In a normal pregnancy, a baby’s pupils become able to dilate and retract in response to light changes at this time. By the 37th week of most pregnancies, the baby is considered full term and labor can begin at any time. By the 40th week, which for most women marks the due date, the baby is ready for delivery.

About this Author

Carolyn Williams began writing for the software industry over 20 years ago. She now writes and edits for online content providers in addition to raising her family. An avid traveler and golf enthusiast, Carolyn delights in sharing her knowledge of destinations both far and near.