For those with baby experience, mothers!

So I'm not ttc nor will be anytime soon, most likely in a year or so. I am 5'9 and weigh 125 and it is quite hard for me to gain weight because of my fast metabolism. I wanted to know, considering my size, if carrying a baby would be hard or what complications or health issues could possibly occur. Obviously our bodies were created to carry children but my fiance brang it up not too long ago and it made me think. Any experiences or fyi's?