Q. LMP was noted mistakenly 10 days later than the actual LMP. Will it affect NT test result?

Answered by
Dr. Sanjay Kumar Bhattacharyya
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Dec 03, 2019

Hello doctor,

My menstrual date was on the 4th of the previous month, and I got married in the same month. This my pregnancy test becomes positive and during my abdominal scan doctor said that my baby's NT rate is 3.17 and the nasal bone is short. Five days later again I have done a scan in that my NT rate came as 2.1 mm and nasal bone is ossified, the result was satisfactory. But mistakenly the ultrasound technician put my LMP date 10 days after my actual date. Without noticing it, we have done a double screening and the result came as low risk. Today I went to the hospital to give a blood sample for triple screening, then only we noticed the mistake on the LMP date of ultrasound. Will it affect my triple screening result? Does it mean my double screening test was wrong?



Welcome to icliniq.com.

I have seen your reports. (attachment removed to protect patient identity).

During the first scan, your baby was 11 weeks and the NT (nuchal translucency) was 3.17. That was a high value but it has been seen in nine out of ten babies with thickness 2.5-3.5 mm at 11 weeks will be normal. The next scan shows your NT was 2.1 mm and your period of gestation 12 weeks. As per your truly calculated fetal age, this was low risk.

At 12-13 weeks, the normal value is 7mm (50th percentile thickness), 2.8mm (95th percentile thickness). So considering these two values only, you are not at high risk.

But I must tell you that the measurement of nuchal translucency alone is less effective for first trimester screening than the combined test (nuchal translucency measurement and biochemical markers). But definitely for this, there is a need to mention the true LMP (last menstrual period). Otherwise, there will be again a mistake and just like the previous one you have to recalculate the whole.

Thank you doctor,

So it means I have to perform an ultrasound again? Will this ultrasound gives me the correct value? What about the triple screening test that I have done today? Will it give a correct value or not?



Welcome back to icliniq.com.

The value of the triple screening test depends on two factors. First of all, the ultrasound age of the fetus and secondly the date of your missed cycle (which is erroneous). The ultrasonologist assess the fetus in contrast with your last date and optimize the fetal age with the help of that value. So, as you can see, everything changes with the date of your last cycle and it is very crucial.

I suggest a back-calculation. If you are found low risk, do not worry. But if you are found in high risk you have to rescore the value again as I have told you. It overtly depends on your last cycle date.

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