It's helpful to know about twins early on, as it's easier to see whether or not they share a placenta.
Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor or midwife to plan your care.
This is even though you don't actually conceive until 14 days after your LMP, or later than this if your cycle is longer than 28 days.
You probably know when your LMP was, but not when you actually conceived.
You may need a second scan to check the first scan's findings.
The dating scan will also show if you're expecting twins, triplets or more.
However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.
Your midwife may recommend an early scan at between six weeks and 10 weeks if you have experienced bleeding.
The person performing the scan is called a sonographer.
If you have agreed to have screening for Down's syndrome, the dating scan and the screening will usually happen at the same time.
Find out more about the combined screening test for Down's syndrome.
Instead, you will be offered a nuchal translucency (NT) scan.
This measures a collection of fluid under the skin at the back of a baby's neck.