What can cause my due date to be wrong?

You have just attended our centre for an early pregnancy scan and your due date doesn’t match your last menstrual period (the LMP) and you are very certain of your dates.

Your due date is important for many reasons. It will allow you to plan ahead for your baby’s arrival and it also helps your doctor know when certain prenatal tests should be performed to make sure your baby is developing normally. With so much being based on your expected due date it’s really important for it to be as accurate as possible. 

A gentle warning is that menstrual cycles are seldom that regular. Normal cycles can be between 21 to 35 days with some women outside of this guideline. Regularity is seldom that common and there is a fair chance the calculation will be off. But we still use your LMP as a guideline because the beginning of a period is an identifiable event where ovulation and fertilisation is not. It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.

An early obstetric ultrasound performed at approximately 8 weeks where a crown rump length (the CRL) is measured precisely predicts an estimated due date accurately. The accuracy of ultrasound measurements used to confirm the calculated due date will be much less accurate further into the pregnancy.

On an early pregnancy ultrasound the embryo’s crown rump length is used to predict your estimated due date ONLY if the measurement falls outside of what your last menstrual period predicts. Measuring the embryo on ultrasound is accurate to within plus or minus five days of the due date. If the crown rump length falls outside of these five days we give you a new estimated due date based on the size of the embryo.