Conception dating ultrasound
If you have an irregular cycle, or if you've recently been taking the contraceptive pill, working out the date from your LMP isn't a very accurate method.
The LMP is used because most women know when their last period was, but may not know when they actually conceived.
If you do know the conception date, this is classed as day 14 and not day one.
Then the circumference of the head becomes the best way to measure your baby Nuchal translucency can be measured from 11 to 14 weeks to estimate the risk of Down's syndrome and other chromosomal abnormalities. Some hospitals offer a nuchal translucency ultrasound to all women at 11 to 14 weeks, but this isn't available everywhere. [Epub ahead of print, accessed April 2009] Gray JAM, Ward P. National Down's Syndrome Screening Programme for England: a handbook for staff.
Sometimes having an ultrasound can cause unnecessary worry. Kettering: National Down's Syndrome Screening Programme.
You should still have a ultrasound between 18 and 22 weeks because more detail can be seen as your baby grows bigger.