When I went to med school we were taught that the B-HCG level doubles every 48 hrs in early pregnancy – and if it wasn’t – you either had an ectopic or a miscarriage until proven otherwise. However, it turns out that this is not the case – just when you thought it was safe to go back to office gynae – along comes some data that completely muddies the waters and makes it all difficult again.
Barnhart et al in 2004 showed the HCG was slower to rise in many women with normal viable intrauterine pregnancies – the 99% confidence for slowest rise (going on to normal pregnancy) was only 53% at 48 hours [eg. from 1000 iu to 1530 2 days later] – tthis is precisely what I thought an ectopic looked like! So I think you need to be careful when interpreting these numbers.
This study out of Oregon in 1990 [Rob Orman was at the peak of his grunge career?] followed a small group of women with high risk pregancies – and 2/3 of the women with ectopics actually had a normal “doubling”.
Kadar et al in 1994 looked at women’s uteri for an intrauterine sac based on both HCG and ‘dates’ and found that ‘dates’ are much better predictors of gestation that HCG. So if you have an idea of LMP or when the HCG first went up – you are better off than using a single HCG.
So that is the bad news – the HCG can really lead you down the garden path if you are not careful. So do NOT rely on a single HCG, be careful of interpreting 48 hour “doubling” HCG levels and if you know the dates- this is actually useful!
The good news – we ED types are actually pretty good when it comes to finding normal IUPs. There was a meta-analysis in 2010 by Stein et al which showed US in ED docs hands was up in the high 90s for sensitivity and specificity for detecting IUP or its absence.
Caveat: you have to practice and get good at interpreting what you see. For a great guide and resources check out Ultrasound Village – their cheat sheets are cool – I use them all the time in my practice.