Studd advocates arms too.
It's do with fat cells so if you apply to thighs it can sit in the fat cells and not fully absorb. Feet are another option and palms of hand, as really good absorption there but not always practical!
The thing is it is not just about absorption though - there are two aspects - how different application sites on body affect absorption (so the aim is to maximise the serum estradiol levels levels from a particular dose), and secondly the safety aspects. The second should obviously take priority over the first.
When most of these products are/were first developed there are trials which include different application sites so - the one I can remember is the Intrinsa testosterone patch as this is the one I read most recently (could look up about the others sometime!) - which showed 25% more absorption of T from the front lower abdomen (tum) vs the buttocks. However I think it was andius which found some info showing the reverse for an oestrogen patch.
The problem is that all of these trials are conducted with very small groups of women who will show wide variation in absorption (this is well known) according to skin, fat etc. As I said before - as long as you are consistent with the location that you use for any transdermal HRT then you will maximise the chance of getting a consistent/stable dose!
I still remain puzzled though as to the discrepancy between the advice for application of oestrogel and the rest of the transdermal oestrogens including Sandrena, in the absence of any explanation as to why, and lack of information on transport through (along) the skin.
If I were to use gel I would always choose Sandrena because it is more concentrated so you need less of it for a given dose of oestrogen.
Hurdity x