Figuring out the due date is easy if you know the exact date of your last menstrual period and when you ovulated. If you have a regular 28 day cycle, you would add nine months and seven days to the date of your last menstrual period. This is the method used by due date calculating tools used in the doctor’s office and found online.
What if you don’t remember the date of your last menstrual period? This is not uncommon. Irregular cycles are another problem with determining the exact due date. In this case, your doctor may suggest an ultrasound to help determine the due date.
Ultrasound is a reliable method of determining the due date, particularly when done in the first trimester. During the exam, the technician will measure what is known as the crown to rump length of the baby. This is the measurement from the top of the head to the rump. The crown to rump measurement is said to be reliable to within three days of the actual due date.
Another method used for determining the gestational age is measuring the gestational sac. This method is used in early pregnancy when the due date is not known. The measurement of the sac is about as accurate as the crown to rump measurement.
In the second and third trimesters ultrasound is less reliable for determining the due date. Depending on the stage of pregnancy, the date can be off as much as one to three weeks. For this reason, if you want an accurate idea of your due date, have the ultrasound earlier in the pregnancy rather than later.
Keep in mind that the due date is just an approximate date and rarely the date the baby will be born. Only about five percent of babies come on their due dates. Most of the rest come within a week or two before or after the due date.
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