Weight issues

Hi everyone. Been worrying about my weight and how it might affect my fertility and wondered if any of you had any knowledge on this subject? I am skinny always have been. Not because I diet or exercise excessively I just am. Despite this I started my periods at 14 when I was even more underweight than I am now and since coming off the pill my cycles are long but regular. I get positive OPK's so I'm hopeful I am ovulating. I just wish I could put on weight ? about 5/6 years ago I weighed about a stone more than I do now and then I had a brace and lost over a stone. Put some of it back on but not as much as I'd like. I worry that I will struggle to conceive and that when I do I may be to underweight to sustain the pregnancy. Just having a night of anxiety overload I think!!