Even though a growing belly might be the only change you notice from the outside, your baby's development changes dramatically over the duration of your pregnancy. Although babies in utero, like those outside the womb, grow and develop at slightly different rates, baby development calendars can provide an idea of the average baby's growth and development during each week of pregnancy. They can also give you some insight into the changes you can expect in your own body.
Significance
When you visit your doctor for your first prenatal appointment, she will help you determine your baby's due date. Doctors usually determine this date by adding 40 weeks to the first day of your last menstrual period, LMP. Although this number is just an estimate of when your baby might arrive---the Kids Health from Nemours site states that most babies arrive between 38 and 42 weeks of their mom's LMP---it can help your care providers track your baby's development in the womb. It can also help you divide your pregnancy into trimesters, a common way to measure the duration of a pregnancy.
First Trimester
The first trimester measures growth from week one to the end of week 12. And, your last menstrual period is counted as part of the pregnancy. So, conception usually occurs around week three and implantation during week four. MayoClinic.com states that the "embryonic period" starts around week five when your baby's spinal cord, brain, heart and other organs form. Your baby continues to grow rapidly during this trimester---developing facial features, arm buds and eye lenses. A couple of other important highlights of this trimester include the beginning of fetal movement in the eighth week---although you will not be able to feel baby move for several weeks yet---and the development of your baby's genitalia in the 11th week.
Second Trimester
In the second trimester, from the beginning of week 13 to the end of week 26, your baby will continue to grow in size and weight. His organs, nerves and muscles will become more fully developed. As his movements grow stronger, you might even start to feel him move. Your baby begins to hear noise around 18 weeks and he might pick out your voice in conversation. Many doctors conduct fetal ultrasounds during this trimester to make sure your baby's grow and development are on track.
Third Trimester
The last trimester---weeks 27 to the end of pregnancy---brings with it the birth of your baby. During this trimester, your baby starts to gain weight rapidly to prepare for delivery. She begins to practice breathing around the 32nd week. You can often see her eyelashes, eyebrows and hair on ultrasound images of your baby. Her lanugo hair, a protective covering that appeared during the second trimester, starts to fall off late in this trimester. Your baby reaches full-term at 37 weeks, which means her organs can function on their own.
Potential
Try not to worry too much about your baby's size as you reach the end of pregnancy. Baby growth varies considerably during this period, according to Babycenter.com, a pregnancy and parenting advice site, and many sizes can be "normal." You should also not worry if your due date comes and goes without the arrival of your baby. Only a small percentage of babies arrive on their actual due dates---about 5 percent, according to KidsHealth. Your doctor will continue to measure your baby and track his weekly growth and development until his birth.
