This measures a collection of fluid under the skin at the back of a baby's neck. Nearly all ultrasound after 10 weeks can be done through your tummy, and most units ask you to come with a full bladder.
Ultrasounds are more accurate than using the LMP and, if the ultrasound date differs from your period dates, the ultrasound date will be used.
The ultrasound date is usually later than the period date, so accurate dating has been found to reduce the number of women having labour induced because their babies are overdue The ultrasound can check that your baby has a heartbeat and is developing normally.
If you do know the conception date, this is classed as day 14 and not day one.
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.
It records high-frequency sound waves that create an image (see Reference 4).
Normally, ultrasounds before until week 4 of pregnancy shows nothing in the uterus and serve no purpose.
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.
The LMP is used because most women know when their last period was, but may not know when they actually conceived.
This is because blood tests for Down's syndrome are not very accurate if you are having more than one baby.
Instead, you will be offered a nuchal translucency ultrasound.
A yolk sac, the early source of nutrients for the embryo is seen around 5.5 to 6 weeks.