But ultrasounds can also lift the lid on a few worries you might be having. The main reason for the ultrasound is to work out how many weeks pregnant you are, and to estimate your due date.The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don't actually conceive until 14 days after your LMP, and later than this if your cycle is longer than 28 days.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.Some parts of popular culture, like baby betting pools, remind us that the date is difficult to guess; however, other messages imply that we can expect a certain date. there is a token message there saying something like “baby’s not here yet?Well, take care of yourself and be patient.” But clearly the message is to expect the baby then, despite the fact that only 4-5% of babies are born on their exact due date.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.
Although their physician or childbirth educator may have told them that the “due date” is only an estimate, many modern American women take it as a scheduled date around which to make their plans.
It will tell you exactly how many weeks pregnant you are. This measurement, known as the crown rump length, is very accurate in the first trimester. National Institute for Health and Clinical Excellence, clinical guideline 62.
After 13 weeks your baby can curl up and stretch out, so measuring the length becomes less accurate. A randomized trial of prenatal ultrasonographic screening: impact on maternal management and outcome. Antenatal care: Routine care for the healthy pregnant woman.
You should still have a ultrasound between 18 and 22 weeks because more detail can be seen as your baby grows bigger.
The dating ultrasound will also show if you're expecting twins, triplets or more.