Which is the real due date?

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It’s very normal to get an estimated due date at each ultrasound. This is not your actual due date.

The most accurate due date is based on 40 weeks lmp(last menstrual period), meaning natural birth typically occurs 40 weeks from the first day of your last period. Fertilization of the ovum (egg) typically happens 2 weeks after that date plus or minus a few days depending on the length and regularity of your cycle. Implantation happens about a week after that.

By ultrasound we can measure your baby and estimate an age, but this is NOT your baby’s age. Based on specific biometrics, like your baby’s head circumference or femur length we can judge what the average gestational age is based on the measurement. This can be helpful when the mother either doesn’t know her lmp or if her cycle is irregular. It can also help your doctor to know if the baby is growing too fast or two slow.

If your baby is big the measurement will say he or she is older than your actual lmp. If your baby is small it will show the opposite.

Sometimes you might get even less precise due date estimates that are based on the height of your uterus as measured with a measuring tape on your abdomen or on the hcg levels in your blood. Again, don’t sweat these estimates.

To check your weeks use this pregnancy calculator. (Hint: For fun tell it that your due date was last year. Keep clicking submit to see all the messages.)

This website cannot substitute competent medical care.

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