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. If your womb (uterus) is very deep in your pelvis, or if you're overweight, you may be offered a vaginal scan, which can get a closer view of your baby.
Last bleeding, to determine if it was a normal menstrual period or lighter bleeding which may have been implantation bleeding.
Plus, menstrual history in general, length of cycles, etc.
On the flip side, if your due date is put too early and you are pressured to induce for whatever reason, you could wind with a preterm baby, which is no fun either.
The bottom line is this- it is worth it to you to put together all the clues you can about your own pregnancy and conception.
The sonographer will put some gel on your tummy and move a small hand-held device (transducer) over your skin to get views of your baby.
It won't hurt, although you may feel a little pressure on your tummy.
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.
Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester.