You'll only be offered an ultrasound when you're between six and 11 weeks pregnant if you have a history of miscarriage, to determine if you are carrying twins, if you've had fertility treatment or you are feeling pain or bleeding, or if you are unsure of the first day of your last period.

The only reason for having an ultrasounds before you're six weeks pregnant is to rule out an ectopic pregnancy. If you have a severe one-sided pain in your belly, it's essential to find out what's causing it. An ultrasound can help doctors decide whether the pregnancy is inside or outside the uterus (womb). Apart from that, it can tell very little else at this early stage. If you just want an ultrasound for reassurance that you have a healthy pregnancy, try to be patient and wait a bit longer!

Usually the early ultrasound will be done vaginally, as your uterus is deep in your pelvis in the early weeks. A vaginal ultrasound gets closer and shows more detail. This type of ultrasound is also better for checking that the pregnancy is inside the uterus. The person doing the ultrasound (sonographer) will be able to see if there are any cysts in your ovaries or fibroids in your uterus, and may be able to find the cause of pain or bleeding.

The later the ultrasound, the more that can be seen:

• At about five weeks (three weeks after conception), it may be possible to see a small, empty pregnancy sac.
• At six weeks, a yolk sac can be seen, and sometimes a heartbeat is visible.
• By seven weeks, the embryo will be about 1cm long, and the heart rate will be about 150 beats a minute.
• At eight weeks, the embryo will measure 16mm, and you might be able to make out the head and body, and some movements.
• At nine weeks, the head, body and limbs can be seen, and it's looking more like a baby. It's very nearly fully formed, and will be known as a fetus.

It's possible to see twins (or more) on an ultrasound from about six weeks, though one baby may be missed at this early stage. Sometimes, a heartbeat is seen in one sac, but not the other. Having another ultrasound in a week or two may reveal a second heartbeat, or the ultrasound may show that one sac is growing and the other is still empty.

It is fairly common for twins to be conceived, but for only one to grow and develop. This is known as the vanishing twin phenomenon. An ultrasound can also tell if twins share a placenta.

Early ultrasounds sometimes show that there might be a problem. If the findings of your first ultrasound aren't clear, you will probably find that everything is okay when you go back for a second ultrasound in a week or two - although the wait may seem very long, and you will naturally be worried.

Unfortunately, though, miscarriage in early pregnancy is fairly common. If an embryo is growing more slowly than normal, or has a slower heartbeat and a large yolk sac, it is more likely to miscarry. The ultrasound may show an empty sac, (a blighted ovum), or a sac with a small embryo, but no heartbeat, known as a missed miscarriage. If you have been having pain or bleeding, you may be partly prepared, but it may be an upsetting time for you.

Because ultrasounds can be inconclusive and not all pregnancies are the same, there are guidelines set out by the Society of Obstetricians and Gynaecologists of Canada for ultrasounds in early pregnancy. If there is any doubt at all over the findings, the ultrasound should be performed vaginally and repeated in one or two weeks.

Reviewed for Canada by Suzanne Wong, MD