Due date disagreement

Based on my last menstrual period, my doctor told me I was 15-week-pregnant. After doing a sonogram, he changed the date, now saying I am only 11-weeks-pregnant, yet I have already felt my baby move. Why are our dates so different?

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Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

During the first trimester, ultrasound evaluation of the embryo is very reliable. It becomes less reliable during the third trimester when the baby is putting on weight but less organ development is occurring.

Quickening, or the perception of fetal movement by the mother, is more subjective. Activity in the bowel can sometimes be misinterpreted as fetal movement. Mothers who have already felt movement in a previous pregnancy are usually able to perceive quickening earlier (at approximately 16-weeks-pregnant) while first time mothers feel it a bit later (at 18-weeks).

Such discrepancy in dates would be unusual if you were very regular (28 to 32 day cycles) and you had no bleeding or were not using any hormonal birth control method.

If a mother is very sure of her dates and she has regular monthly cycles, most providers try not to change due dates based on one ultrasound. However, with a large discrepancy -- such as yours -- most providers would probably change a due date.

Ultrasound is only as good as the equipment, the technician and the interpreter. It is possible that a mistake has been made. Or perhaps a previous pregnancy was lost and a new one has begun. You may wish to request another ultrasound in a month or six weeks, especially if your uterus is not growing in accordance with the new estimation.

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