However, I was concerned that if the elastic was tight enough to hold the trousers up early in the pregnancy, it would be too tight later on.
To find out how I have solved this, read on.
So I invented a method to get round this. (When I say invented, I mean I had to think this out for myself, I hadn't read the idea anywhere else. It's not to say someone else didn't invent it as well!) It has resulted in a band which was snug enough to hold up only a few months in, but will hopefully continue to expand as she does, without becoming constricting or uncomfortable.
I used a piece of 'normal' elastic (about 3/4" width) round the back, measured as a comfortable length to go round the back of her waist. Having measured her back waist, deducted a couple of inches or so, to allow for the fact that you need a bit of stretch. I then joined it to a piece of buttonhole elastic for the front, the same width (about 3/4"), this I left fairly loose (i.e. longer than the back. I threaded it through the waistband, having left a slit in both side seams of the waistband, and joined the other ends of the two elastics together. I finished the edges of these slits but left them open. I then sewed a button on each side underneath the elastic, so that the button hole elastic could be pulled up tighter to start with, and fastened onto the buttons, and then gradually released as the tummy expands. The idea is to button the elastic twice, once, near the end to stop it slipping back through the waistband, and the second time, to hold it at the length you want for a comfortable fit. It can be adjusted at both the left and right hand side.
This seem to have worked well, so I also did the same on other pairs of trousers I made from a modern maternity pattern that made no mention of elastic, but which were very loose at the waist and which tended to slip down rather embarrassingly.
Of course, you can use this method on any pair of trousers for which you want to allow waist expansion, for example for a child. You wouldn't then need to fix the back waist stretch, as you'd want to waist to stretch evenly all round. You would just pull up the elastic as much as required. And you may just be able to put the buttom hole elastic just on noe side, or at the back, with a msaller waistband.
Incidentally, it would be possible to use button hole elastic all the way round, so you only had one join in the elastic. I didn't do that for two reasons. One, it tends to be more expensive than ordinary elastic, so I chose to use it sparingly as needed. The other more important reason, though, is that I have generally found it to be more stretchy (looser) than normal elastic and I thought it might stretch too much on the back. As it happened, this wider button hole elastic was pretty similar stretchiness-wise to the normal elastic, so I don't think I gained much, as it happened.