20 Weeks Short Femur Length


Dear Ms. Ultrasound,
I recently had my 20 weeks ultrasound and need some advice. The doctor informed me that my daughters' femurs are measuring small for her age. He said that he wasn't 'overly concerned' but thought we should get it checked out.

At first, I didn't think that much about it because he didn't seem to act like it was a big deal. After I left the office and news started to sink in, I became scared. I called the office back and talked to the nurse who could not get me exact numbers at that time (the ultrasound information was already in transit to the scanning area) but she could tell me what the doc had listed in the computer.

Both of the femurs are the same length but measuring short at the 2.3% for 20 weeks. When I was looking for size charts it seems like there is only a 3mm difference in length from the 50% and the 5%.

Is this difference a major concern? From what I heard from the doctor everything else at that time of the ultrasound appeared to be normal. I don't believe they measured the arms as well as the legs at this time.

What are my chances that the measurements are just incorrect? I am obese and the nurse did tell me that their machines weren't very good for someone of that size and therefore not as accurate as the machines they will use at my specialist appointment.

Could it also be that I am just not as far along as I had thought? When we had our initial 8 week ultrasound, she was measuring 5-6 days later then I had figured based on my cycle.

I know there are other worse things to consider but I'm trying to see if there is reason for me to remain optimistic.

Thank you for any information you can give me. I feel like I'm still really in the dark on whether it's a concern as an abnormality or if she is more likely to be just fine.



Please don't worry too much about this inconsistency. This is the one measurement that can be way off because of technical factors.

Because the facility where you had your ultrasound did not have up to date equipment and the fact that you said you are obese, the measurements may not be accurate. The femur measurement changes significantly with just a few millimeters. This means if the resolution of the ultrasound equipment was not good, it may have been difficult to assess where the end of the femur was.

Once you go to the specialist, he/she will give you a much better idea of how well the baby is growing and if all the measurements are within the normal range. The anatomy commonly measured is the diameter of the head, the circumference of the head and the circumference of the abdomen.The arms can be measured but it is not the routine.

For now I would try and let go of too much worry. Just connect and bond with your baby. I feel it is very important.

Don't forget to ask a lot of questions at the next exam. Sometimes they wont tell you results during the ultrasound, but when you go to the doctor "ask,ask,ask"! Don't leave until you have a full understanding of what's going on.

Write back after your next ultrasound if you still have questions.

With regards,
-- Jane RDMS

Jane Foley

Jane Foley has worked as a Sonographer (Ultrasound Technologist) since 1979. Jane has lived and worked in many parts of the world including Saudi Arabia. She is a Registered Diagnostic Medical Sonographer or RDMS. She pulls a wealth of information from her experience in the field of Radiology and her interactions with such a broad cross-section of cultures she has visited. She now makes her home on the island of Maui with her English husband, Michael.