Discrepancies of a Late Pregnancy Ultrasound

Jane Foley's picture


Dear Ms Ultrasound,
Hi,My wife is now 39 weeks pregnant and doing well. However at her routine 37 week check up her doctor decided to do an ultrasound to determine if the baby's head was in the pelvis or down position. He is.

He then started taking measurements of the long bones and became concerned that they were too small. He sent my wife the high risk doctor to do a more extensive test. They all came back as saying the bones were looking on the small side.We have been told by the doctor and many previous mothers that an ultrasound at this stage is way too subjective and the data can be off. Is that true? Because of the size of the belly, couldn't the reading be off because of the position of the baby?

Basically I would like to know how subjective are these tests late in term because we have been stressing over this issue. We were told for 36 weeks that everything was OK, then our doctor does a late ultrasound and worries us beyond belief. We feel in our hearts that everything is fine and they are covering their own butts in case they are right, but we are just looking for some outside advise on the discrepancies of a late pregnancy ultrasound.

Thank you,


Hi Michael,
My hopes and experience tells me the baby should be fine. There are many degrees of limb shortening a fetus may experience. The earlier limb shortening is recognized, the worse the prognosis. The ultrasound on your wife was done in the late 3rd trimester after normal, earlier ultrasounds, so this falls into the 'good outcome' category.

The last few weeks of fetal growth can result in a wide scale of measurements. On the machine I use, the measurement package always seems to cause the femur length to come up short. Also the placement of the caliper moved a couple millimeters can result in a shorter femur length. So yes, this measurement is subjective and it is quite normal for all the measurements to vary up to 3 weeks difference in the last few weeks of a pregnancy.

It seems your doctor has taken the appropriate actions, and has sent you to the prenatal specialist. From what you stated in your letter, the results did not seem to worry anyone too much, otherwise they would have sat you down and explained more than "looking on the small side."Finally, it is so scary for Mom and Dad but necessary when a measurement falls just outside normal. Hopefully in your case all is well. If you get a chance drop me a line and let me know what the outcome was.

-- Jane, RDMS