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.
Ultrasounds are more accurate than using the LMP and, if the ultrasound date differs from your period dates, the ultrasound date will be used.
As with all screening tests, there can be false positive and false negative results. You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP). However, conception doesn't actually occur until around 14 days after your LMP, or later if your cycle is longer than 28 days. Multiple pregnancy: antenatal care for twin and triplet pregnancies CG129. 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.
Knowing about twins early is also useful if you want to have screening for Down's syndrome.