12 week dating scan accuracy

Some parents-to-be are desperate to find out the sex of their unborn baby as early as possible, and will search for clues at the very first scan. Everyone knows you can't tell the baby's sex at the 12 week (or there abouts) dating scan. Well, some people think you can, thanks to a little known phenomenon called the 'nub theory'.

The nub theory works on the basis that from 12 weeks, boys' genitals (or the genital tuber, as they are not actually properly shaped bits and pieces at this stage, and both sexes will have a little tube- like protrusion) will be angled differently to a girls', and so by carefully examining your scan picture, you can work out the sex.

Some babies may survive to adulthood, but this is rare.

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This is to assess your chances of having a baby with these conditions. Edwards' syndrome is also called trisomy 18 or T18, and Patau's syndrome is also called trisomy 13 or T13.

If a screening test shows that you have a higher chance of having a baby with Down's, Edwards' or Patau's syndromes, you'll be offered diagnostic tests to find out for certain if your baby has the condition.

If it wasn't possible to obtain a nuchal translucency measurement, or you're more than 14 weeks into your pregnancy, you'll be offered a test called the quadruple blood screening test between 14 and 20 weeks of pregnancy.

This only screens for Down's syndrome and is not quite as accurate as the combined test.

The screening test will not tell you whether your baby does or doesn't have Down's, Edwards' or Patau's syndromes – it will tell you if you have a higher or lower chance (also called higher or lower risk) of having a baby with one of these conditions.

If you have screening for all of them, you will receive two results: one for your chance of having a baby with Down's syndrome, and one for your combined chance of having a baby with Edwards' or Patau's syndromes.At the scan, the fluid at the back of the baby's neck is measured to determine the "nuchal translucency".Your age and the information from these two tests is used to work out the chance of the baby having Down's, Edwards' or Patau's syndromes.For example, you may be offered diagnostic tests that can tell you for certain whether the baby has these conditions, but these tests have a risk of miscarriage.You do not need to have this screening test – it's your choice.Some people want to find out the chance of their baby having these conditions while others don't.

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