The scan is carried out in a dimly lit room so the sonographer is able to get good images of your baby. You'll then be asked to lower your skirt or trousers to your hips and raise your top to your chest.
The sonographer will put ultrasound gel on your tummy and tuck tissue paper around your clothing to protect it from the gel.
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The gel makes sure there is good contact between the machine and your skin. It is this probe that sends out ultrasound waves and picks them up when they bounce back.
A black and white picture of the baby will appear on the ultrasound screen.
If a woman has had unprotected sex with more than one man, she may want to use the conception calculator to give her an idea who the father may be.
However, if there is no menstrual cycle between the bouts of unprotected sex, a conception calculator will not effectively reveal paternity.
for more information on what may happen if a scan or other screening test suggests an abnormality.
Most scans are carried out by specially trained staff called sonographers.
As part of your pregnancy (antenatal) care, your midwife will offer you a dating scan that will give you a more accurate date for the birth of your baby.
Find out about what's happening when you're 4 to 8 weeks pregnant.
The scans are painless, have no known side effects on mothers or babies, and can be carried out at any stage of pregnancy.
Talk to your midwife, GP or obstetrician about any concerns you have.
To find your due date, enter the date of the first day of your last period, and click 'show your due date' - the calculator will do the rest.