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Is it a problem if the femur of the fetus is small?

Friday, 02 July 2004
Answered by: Dr. Puneet Bedi
Consultant Obstetrician and Gynaecologist,
Indraprastha Apollo Hospitals,
New Delhi
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Q. I am 23 weeks pregnant. I did a scan and the doctor told me that the femur of the fetus seems to be small. Is it a very serious thing? Please let me know about it.

A.  Your question can only be answered if you understand how the femur of the baby is measured and why. The femur is measured with an ultrasound. The measurement at best is approximate and the error can be plus/minus one mm (2 mm range) in the best of hands (the error is much more in inexperienced hands). Now these mm measurements are put against standard charts in the ultrasound machine software which is almost always calibrated against western standard measurements. As our babies are smaller than western babies there is bound to be a margin of error there as well. This figure thus measured in mm is converted into weeks(by western charts). Hence the estimations are at best approximate. Now that you understand the limitations of these measurements should you be bothered about whether the figure for femur length given to you is normal or not? Well no you should not be, nor in my opinion should the ultrasound doctor be sounding so worried. As two kids of the same age would vary in height and weight so would two babies of 22 weeks be. And only if all these considerations of natural variations are taken into account should a femur be called short. In general your height and that of your husband will decide whether your baby will have long or short femurs. In general if the discrepancy is more than three weeks should you be worried. (meaning at 22 weeks the femur length is less than 19). Even if that is so in all probability it is a short normal baby but you should get a chromosomal analysis done by fetal blood sampling and be in consultation with a fetal medicine specialist. From what you have written it does not seem to be an abnormal baby.

A.  Your question can only be answered if you understand how the femur of the baby is measured and why. The femur is measured with an ultrasound. The measurement at best is approximate and the error can be plus/minus one mm (2 mm range) in the best of hands (the error is much more in inexperienced hands). Now these mm measurements are put against standard charts in the ultrasound machine software which is almost always calibrated against western standard measurements. As our babies are smaller than western babies there is bound to be a margin of error there as well. This figure thus measured in mm is converted into weeks(by western charts). Hence the estimations are at best approximate. Now that you understand the limitations of these measurements should you be bothered about whether the figure for femur length given to you is normal or not? Well no you should not be, nor in my opinion should the ultrasound doctor be sounding so worried. As two kids of the same age would vary in height and weight so would two babies of 22 weeks be. And only if all these considerations of natural variations are taken into account should a femur be called short. In general your height and that of your husband will decide whether your baby will have long or short femurs. In general if the discrepancy is more than three weeks should you be worried. (meaning at 22 weeks the femur length is less than 19). Even if that is so in all probability it is a short normal baby but you should get a chromosomal analysis done by fetal blood sampling and be in consultation with a fetal medicine specialist. From what you have written it does not seem to be an abnormal baby.

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