Using a Pregnancy Calculator
Your due date is important, not only so you know when to have the nursery ready, but so you'll know how far along you are. This helps you track the progress of your pregnancy, to ensure the baby is growing and developing on schedule. To estimate your due date, you'll need some sort of pregnancy calculator.
If your periods are fairly regular, the task is straightforward. The average human pregnancy lasts about 280 days (40 weeks) from the first day of your last period. Grab a calendar, count forward 40 weeks, and circle the date. Alternately, take the date of the first day of your last period and add 7, then count back 3 months. For example, if your last period started on May 2, 2005, add 7 to get May 9. Going back three months gives you a date of February 9. Your due date is February 9, 2006. Or visit one of the many Web sites that offer an online pregnancy calculator. Simply plug in that all-important first day of your last period, punch a button, and receive your due date.
If your periods are irregular, though, a regular pregnancy calculator might not be helpful. Then it becomes time to turn to other methods. If you should happen to know the exact or approximate date of conception, the date can be calculated from there (count forward 38 weeks in this case). Or your practitioner could order an ultrasound. This can provide measurements of the head diameter, the head circumference and the abdominal circumference, which can then be used to determine gestational age (how long your baby has been in the uterus). Ultrasound to determine gestational age is most accurate when done between the 8th and 18th weeks of pregnancy.
Another pregnancy calculator is through measuring the size of the uterus. This is done through external palpation at your prenatal exams. After about the first trimester, your practitioner can also measure the height of the fundus (the top of the uterus), which usually corresponds with gestational age. However, growth spurts and individual variations in development make these methods less accurate in determining your due date.
Of course, only about 5 percent of women deliver on their projected due date. So don't rely too strongly on whichever pregnancy calculator you use, because in the end, this is your baby's call.