Which Method of Calculating My Due Date is Most Accurate

Jane Foley's picture


Hi! I am just wondering if the LMP method or U/S method is more accurate? My menstrual cycle is very irregular. Ultrasounds done at 10 weeks 1 day and 16 weeks 4 days gave the same due date. I recently did a 4d U/S at about 28 weeks and it placed my due date four days later.

Which is more accurate -- the LMP, ultrasound or the 4d ultrasound?


The Last Menstrual Period (LMP) method of dating a pregnancy is a good starting point for a woman and her doctor. It assumes that every woman has a 28 day cycle and that she becomes pregnant on day 14.

If you have regular monthly periods, the ultrasound and the LMP are usually very close. Of course this does not apply to every woman, especially someone like yourself who does not have regular periods.

The ultrasound is an important part of dating a pregnancy when a woman has irregular menses. It should be done in the first trimester when it is the most accurate.

Let's look at your stats. Your first ultrasound done at 10weeks 1 day will have given you and your doctor a due date which you should carry throughout the pregnancy. Any ultrasound done later should not change your due date. As the pregnancy progresses, the dating of the pregnancy becomes less and less accurate. You may be a week or more different in the second trimester with normal growth.

As the fetus grows, considerations must be made for each mother and her clinical and genetic profile. I might give birth to a nine pound baby and you a six pound baby. If we both had an ultrasound during the third trimester we would get different due dates. In the first trimester, most fetuses are the same size no matter what.

So, in answer to your question, the ultrasound is the most accurate. The 4d has nothing to do with dating, the date done is the key.

With regards,
~Jane Foley, RDMS