Obstetric dating accuracy
Hormones in your blood vary according to your stage of pregnancy.
A blood test at what you thought was 13 weeks wouldn't be helpful if you were actually 16 weeks pregnant.
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.
It shouldn't be uncomfortable, and you don't need a full bladder Your sonographer will give you a report of the scan, which will tell you exactly how many weeks pregnant you are.
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.
It's helpful to know about twins early on, and it's easier to see whether or not they share a placenta during the first trimester.
Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor and midwife to plan your care. Nearly all scans after 10 weeks can be done through your tummy, and most units ask you to arrive with a full bladder.
This means that accurate dating reduces your chance of having your labour induced because your baby is thought to be overdue.