My BMI is 38 too (well I think it's 37.8 but they round up). I was told I was allowed to gain no weight at all from my 10 week booking apointment, I have gained about 2lbs and I'm seeing the consultant next week and I know she'll be angry. They said if at the time of giving birth I'd gained a stone I'd be better to have an epidural which I absolutely do not want. I think it's a little unfair to tell you not to gain weight and then not give you any idea of how many calories to eat, or weigh you inbetween. I haven't eaten any more in the third trimester but have gained weight because little man has. I have no other pregnancy problems except BMI. My early scan said view restricted due to BMI, the others haven't but they've never had to rescan or not been able to see anything.
My BMI is now approx 39, and was almost 42 when I got pregnant.
I've been seeing a dietitian since week 6, and while I've actually not gained any weight (I lost 13lbs since then), she keeps reassuring me that it's ok for me not to have gained any weight, and at the same time, she also told me right from the start that it's theoretically ok (and I should expect) for me to gain anywhere between 6-11kg (13-23lbs) by the end.
I think it's ridiculous that they would tell you that you have to have an epidural if you gain weight. I've read that epidurals are less ideal for overweight women, because the recovery is far harder because there are more layers to cut through when they make the incision.
I've not had any troubles with my US scans. The doc usually asks me to pull my tummy up, and on the early anomaly scan I had he switched to vaginal part way through in order to be able to see better (which I actually prefer because it was less uncomfortable than the external scan).
When I went to the dietitian, she gave me general guidelines of what to eat, with a general menu, and told me what to steer clear of, which was a huge help. She also gave me a manual of foods and their equivalents, as well as a guide to eating healthily during pregnancy. They're amazing here, and want to do everything they can to help you.
I've been going to her every two weeks, and she weighs me and puts it into the computer, so they can keep track.
!) then I had a very straight forward birth with no complications caused by my size. Only complication was meconium... can happen to anyone.
) BMI at 20week app was 39.7 and i've not gained any weight since 8week app 
here's my latest compare pic 
