A ‘dating ultrasound’ or ‘dating scan’ is a scan done early in pregnancy (before 13 weeks). It is usually done to check how many weeks pregnant you are and therefore to calculate the estimated due date, but it can be done for other reasons.
However, some women do need a scan before 13 weeks. Between 8 – 13 weeks of pregnancy is a good time for a dating scan.
Why might I need a scan before 13 weeks?
A scan before 13 weeks may be needed if you:
Are uncertain about your last menstrual period (LMP) or if you have a very long or irregular menstrual cycle.
Became pregnant while taking the oral contraceptive pill or other hormone medication.
Are having bleeding in the pregnancy.
Have an important medical illness which could affect your pregnancy like diabetes, epilepsy, or major heart disease – having an accurate due date can help plan the pregnancy.
Have had a previous miscarriage - while it is not necessary to have a scan if you have no bleeding this pregnancy, many couples find it reassuring to see the baby and the heart beat at around 8 weeks. If everything is looking good at 8 weeks, the chance of miscarriage drops to very low (although not quite to zero).
Have had a previous ectopic pregnancy (pregnancy in the tube or other location outside the uterus) – an early scan will let you know if the pregnancy is in the right place this time. An early scan generally gives much less information than a scan at 13 weeks or later. However, very early scans may occasionally identify:
A serious abnormality – of course, that most abnormalities are not visible until 12 weeks or often much later. Some are not visible until long after birth.
A twin pregnancy – and also identify whether the twins will have two placentas or will share one
An early scan generally gives much less information than a scan at 13 weeks or later. However, very early scans may occasionally identify:
A serious abnormality – of course, that most abnormalities are not visible until 12 weeks or often much later. Some are not visible until long after birth.
A twin pregnancy – and also identify whether the twins will have two placentas or will share one placenta, which is important to know.
How is the early pregnancy scan done?
Generally, the most important thing when doing an ultrasound is to try and get a very clear picture In early pregnancy, that means the ultrasound can be done in two different ways – though the stomach or through the vagina. Which way is better depends on a number of things including how far along the pregnancy is.