In the past, most women didn't learn that they were pregnant with twins until they were in labour. Today, with modern techniques such as ultrasound such late surprises are rare. Couples typically discover they're having twins during a routine ultrasound, often done in the first trimester. Ultrasound testing is almost foolproof when it comes to diagnosing twins, but the greater the number of foetuses, the less accurate it becomes.
It's possible to see twins (or multiples) on an ultrasound scan at around six weeks, although one baby may be missed at this early stage. Sometimes a heartbeat can be seen in one amniotic sac, but not in the other. A second heartbeat may be revealed in a rescan in a week or two, or the scan may show that one amniotic sac is growing while the other remains empty. This is a fairly common, naturally occurring event known as the vanishing twin syndrome, where twins are conceived but only one will develop and grow.
If your pregnancy is a result of a fertility treatment such as in vitro fertilisation (IVF), you will most likely have an ultrasound as early as four or five weeks into your pregnancy, since in most cases there will be more than one embryo implanted to ensure success.