A multiple pregnancy is confirmed by an ultrasound scan, usually during the first trimester. The first trimester scan is an important assessment tool when you are expecting multiples because this can be the easiest time to identify exactly what type of multiples you are carrying, and how their amniotic membranes are set up (chorionicity), since this can have major consequences later in pregnancy.
In a very early scan, the sonographer will be looking for the yolk sac (which disappears gradually as an embryo develops) and whether there is one or two—usually, if there are two yolk sacs, then there will be two amniotic sacs. Between 10 and 12 weeks, they might also look for an amniotic membrane, which can be very thin and extremely hard to see on an ultrasound.
At this point the sonographer should be able to identify if there are two separate sacs, each containing a baby—or if there is a more complicated kind of arrangement, such as one sac containing two babies (monochorionic twins).
If the first scan is done before 15 weeks, it will have nearly 100% accuracy in identifying chorionicity, and the sonographer should be able to tell you this—if they don’t, you need to follow this up with your doctor or midwife. After 16 weeks, the babies are too big to accurately find out the chorionicity.
So, at your first scan, it is very important that you find out what type of multiples you are carrying, as this will affect the kind and level of monitoring you need throughout your pregnancy. The sonographer who does your scan will be looking to see if there is one placenta or two, and how many amniotic sacs there are. These factors are an indicator of increased risks to the health of the mother and her babies.
For more information on determining chorionicity, click here.