thanks hellkat. just found this on babycentre:
What can be seen on a scan in the first trimester?
The only good reason for having a scan before six weeks is to rule out an ectopic pregnancy. If you have symptoms of an ectopic, such as severe one-sided pain in your lower abdomen, an early diagnosis is vital. A scan can help doctors decide whether the pregnancy is inside or outside the uterus. Apart from that, it can tell very little else at this early stage. If you just want a scan for reassurance that you have a healthy pregnancy, try to be patient and wait a bit longer!
At five weeks (three weeks after conception)
The sonographer may see a small gestation (pregnancy) sac that looks like a black hole because it is filled with fluid. She may not see anything else yet, so will probably ask you to come back for another scan in one or two weeks. The diameter of the gestation sac is used to calculate your dates until an embryo is seen.
At six weeks (four weeks after conception)
The yolk sac (this is attached to the embryo and contains nutrients to feed it while it is developing) may be seen as a small white circle in the gestation sac, and a heartbeat can be seen in about 50 per cent of pregnancies. This means of course that in the other 50 per cent, a heartbeat WON'T be seen. You may then worry that something is wrong, when in fact the appearances were normal for that stage.
At seven weeks
A tiny embryo with a heartbeat can usually be seen.
At eight weeks
The developing embryo measures about 1-2 cm and should now be easier to see on the scan. It will grow very quickly, doubling in size every couple of weeks.
At 10 weeks it measures 3 cm.
At 12 weeks it measures 5-6 cm.
At 13 weeks it measures 7 cm.