It is only under control of ALLAH to give child to a couple. medical science is only a supportive entity and nothing else. keep faith on ALLAH. do intercourse on 11, 13, 15, 17 days of ur menstrual cycle.
for weight gain, do regular walk and dont eat oily and fatty food. drink plenty of water and add fruits and veggies in ur diet.
if u are very much worried, get a complete check-up done by gynacologist.
also write ur span of marriage.