Calculating Due Dates

How do you go about Calculating Due Dates?

Calculating due dates is a frequent question for mothers to be who want to know when the baby is due so they can plan baby showers and other things accordingly.  Due dates are based on the pregnancy gestational age which is calculated from the first day of your last menstrual period.  The exact date of conception is hard to determine so due dates are generally calculated from the first day of the last menstrual period.

Calculating Due Dates:

First Day of LMP: The first day of your last menstrual period is a good guide to calculate due dates for moms-to-be who have a fairly regular period.  This basically works by starting from the first day of the last menstrual period and going out 40 weeks.

Sonogram: A sonogram can be used during the early part of pregnancy to help calculate due dates.  Songrams are fairly accurate in helping to determine the due date, but they do become less accurate as the pregnancy progresses.  Sonograms are frequently used between the 8th and 12th week of pregnancy to evaluate the progress and gestational age of the baby.  Accuracy is enhanced when the first day of the last menstrual period is known along with an early sonogram.

In Vitro Fertilization: A mom-to-be who has become pregnant through in vitro fertilization or other assisted reproductive technologies will usually have a solid expectation on when the due date should be.

When is the expected due date?

Estimated Due Date: Due dates are only expected or projected dates for delivery.  Development is fairly consistent but the actually due date of a full term pregnancy may vary a week or two in either direction.  Less than 10% of all births actually occur on the specific due date.  Your healthcare provider will monitor the development and wellness of your baby and there is little concern if the baby comes a week early or a week late. Due dates are only estimates.

What factors hinder calculating due dates?

Last Menstrual Period: An irregular period, not tracking your menstrual period, or forgetting when your period occurred can all be factors that make calculating due dates more challenging.   If you can estimate when your period started and combine this with a sonogram, it should provide a fairly accurate due date – at least enough for you to plan your prenatal visits, showers and other pregnancy related activities.

Baby’s Growth: In some cases it is difficult to determine the gestational age because the baby is unusually large or small. Also, in some cases the size of the uterus in early pregnancy or the height of the uterus in later pregnancy does not match the first day of the last menstrual period. In these cases as well, it is difficult to obtain an accurate gestational age.

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