many people have doubts regarding the 3rd month NT scan ad blood test report I will clarify the two most common doubts in this video many patients ask me In which exact week should I do this scan as 3rd month consists of so many weeks like 11, 12, 13, etc hence, when should I come for NT scan I would say only one thing we should get the NT scan done when the baby is about 4.5 to 8.5 cms but if you ask about the ideal time for scan the particular length which should be perfect to access the baby is 5 to 7.5cms because the baby will be perfectly structurable for us to see the baby will be too small when it is 4 to 4.5 cms the baby will not in position when it is about 7.5 to 8cms so the baby will be sizable when it is 5 to 7.5 cms and so we will be able to access the nuchal fluid, nasal bone and the other structures so the baby will reach that level at 12 weeks hence it will be ideal to do the scan during 12th to 13th week for your doctors to screen though you can do it during 11+weeks or 13+weeks but if you wish to go during the perfect time then 12 to 13 weeks will be ideal one more question doctor, my scan is normal, then why should I take blood test why should I do blood test when my scan is normal but what you should know is in an NT scan we can see only the structure of the baby in any scan we can see only the structural anatomy of the baby in the blood test, what do we see in the blood which we take from the mother we are indirectly checking the hormonal level of the baby during pregnancy, the baby and the placenta produces two hormones there are many other hormones, but we check the two specific hormones in this blood test HCG and PAPP-A those two hormones are very important because down syndrome babies that is chromosomally abnormal babies will have high HCG level and low PAPP-a level for some other abnormalities, both the levels will be low so if we check all these levels we will be able to say with 85 to 90% accuracy that is scan along with blood test if you do only the scan, then the chances of screening is about 70 to 75% when you include blood test, the chances of accuracy is up to 90% so that is the reason why your doctor will advise for blood test once you have a normal NT scan