Renaee, if you're going to only have one, have the 20w one. It picks up if the organs are formed and if they will work when the baby is born, shows that the spine and brain have formed correctly and also shows that the other bones (such as the leg bones) are all present and correct. OK, you're not going to terminate - but what if the scan picks up that your baby will need surgery either in utero or straight after birth? Yes, it is traumatic to find that out, but if your baby has a serious problem then the scan can pick that up and it can be treated as early as possible to give your child the best chance of survival.

I agree that scans shouldn't be done on a whim, or to get a half-hour video, but a quick 5-minute scan (maybe 10 mins at the 20w one if the baby is being "awkward") won't do any harm. If you wanted a half-hour scan every day then, looking at the studies, something bad may happen, but there's only a chance of that: less than an hour of scans over 40w is not going to hurt.