OK- IAAD, and I started an O&G job two weeks ago, so I can give you some information, but this should in no way be taken as medical advice etc. etc.
Yes, a scan at 6-7 weeks should tell us if we’re looking at a twin pregnancy. If they didn’t see a second baby, there isn’t a second baby to see.
Relying on HCG alone is not a good idea.
It’s not reliable as a one off measurment, and waiting 48 hours to get 2 in a row is much less convenient and reliable than doing a Transvaginal ultrasound scan (TVUSS).
TVUSS shows things on average 7-14 days earler than an abdominal USS, and although the probe looks remarkably like a curling iron, only the first 2-3cm actually goes in, so it isn’t as horrible as it first appears.
You should see gestational sacs from about 4 weeks.
If the sac is over 20mm your operator should see a foetal pole.
If the foetal pole is over 6mm, your operator should see a foetal heart.
If the sacs or foetus(es) are smaller, we are not able to confirm a viable pregnancy- usually we advise women to return for a further scan in a week to 10 days.
2 sacs, 2 babies, 2 heartbeats= twins (possibly identical, possibly fraternal)
1 large sac, 2 babies, 2 heartbeats= identical twins
2 sacs >20mm, 1 baby, 1 heartbeat= one pregnancy and either a clot above the sac giving the appearance of a second sac, or an early miscarriage of a second twin.
A significant proportion of people found to be pregnant with twins before 8 weeks will go on to have only a singleton pregnancy. In other words, the earlier you see twins, the more likely it is that the person will not go on to have twins.
The hospital where I work has a daily walk-in Early Pregnancy Assessment Clinic, for anyone in the first 12 weeks with any concerns (cramping, bleeding, hyperemesis, reassurance after previous early miscarriage etc). We give everyone who attends a TVUSS, and advise women who are found to have two gestational sacs before 8 weeks that there is a chance that one of the pregnancies won’t continue. If we’ve seen 2 healthy heartbeats, the chances smaller that we will lose one of the twins.