You don't have to put on a ton of weight you know! In fact it's really not a good idea to get too big.
I was advised by the hospital dietitian that all I needed extra in my diet was the equivalent of a chicken sandwich, (400 cals) but only on the last trimester, and breast feeding, and to try not to go mad!
Fwiw I put on two and a half stones, and was a stone heavier after having my babe. I breastfed and I gradually lost half a stone without thinking about it much. I was 8stone at three months pregnant, (had morning sickness) and 10 stone at full term, 9 stone after 6 months pp.
I only started to show at 6 and a half months, at 7 months I had quite a big bump, and at 8 and a half months I was absolutely huge, got smaller towards the end as babe dropped down. I was terribly bloated with water during the last month- my feet were huge - I put this down to eating too much wheat in the last month, as I suddenly craved cake.
Very different shape now though, (emergency c-section, and split abdominal muscles) and I would buy a corset now, if I got pg again, to cinch myself in afterwards!!
In some ways it doesn't really help to beat yourself up over any weight gain, or lack of it.
It's a bit of a rod to beat women with tbh- A lot of 'weight' is water retention.
Just make sure to take a lot of folic acid in the first part, and before you get pregnant, to build up your stores.