Loosing weight to have a baby.

So recently I've been really unhappy with my weight, always been heavier but the last 3 months I've been really trying to do something about it. Mainly because I want to have a baby. 

I told my sister that my plan was to lose weight so I could have a baby, and she laughed and thought I was mental! 

I'm a size 16/18 and I'm 5ft9 so it's all pretty evenly spread lol and some people don't realise that I'm actually as big as I am, but I have slightly irregular periods and I think that might be due to my weight and possibly make it harder for me to conceive because I don't know when I ovulate properly and stuff. But my boyfriend agreed with me (in the nicest way) that he thinks it would be safer for me and our future babies health if I lost some weight and my sister basically said he's being selfish and mean and he's saying it so he can have a skinnier girlfriend. I don't believe this because he's been with me at my biggest and even then he told me he wanted to have a baby with me. But my sister keeps taking the piss saying it's stupid to lose weight for a baby when I'm just gonna put on weight while pregnant and shit. 

Am I being stupid? I want to lose weight anyway for myself but I can't help feel stupid about it. 

Has anyone else lost weight to try for a baby?