Twin pregnancy weight gain question

Hi Everyone. Just curious how much weight has your doctor instructed you to gain? Mine seems excessive..I was told to gain 50 pounds! So far I've gained nothing. Just not hungry. He said the more weight I gain, the better chance that they wont come early. Is this true?with my son's pregnancy I gained nothing and he was 9 pounds and healthy. I'm not opposed to gaining for the sake of the babies but seeing as I get full so quickly and am not hungry to begin with it just seems impossible to gain 50 pounds in 5 months.