A due date is, in reality, an educated guess by your physician based on your recollection of dates and, later during pregnancy, of your baby’s growth. The fact is your baby will arrive whenever she’s ready to arrive, not before or after. Most births don’t actually occur on the due date, believe it or not.
A healthy pregnancy is considered anywhere from 39 to 42 weeks. (Although full term is considered 37 weeks, many experts believe that babies have the best results in terms of health when delivered after week 38).
The due date prediction is fairly straightforward. Go back to the first day of your last menstrual period. Pregnancy typically lasts 280 days from that time until delivery. If you usually have a longer cycle than 28 days, you might deliver after your due date, and if yoru cycle usually is shorter, you might deliver a few days prior.
In addition to this sort of calculation, your health care provider might use other means to try to predict a more accurate due date. Using an ultrasound, your health care provider will take a measurement of various parts of your baby’s growth and development. The size of your baby during these ultrasounds, as well as the growth that your baby makes between ultrasounds, might prompt your health care provider to alter your due date, if the measurements don’t match up very well with the date calculated using the date of your last menstrual period.
Once you hit 40 weeks of pregnancy and become “overdue,” your health care provider will probably suggest some stress testing in order to make sure that your baby is still doing all right. If pregnancy goes much past your due date, your health care provider may suggest that you should have labor induced. On the other hand, if you were to go into labor before the 37th week of pregnancy, it is likely that your health care provider would like to be able to stop labor, and postpone it long enough for your baby to do some more developing.