Placenta / Umbilical cord

My baby girl is on breech side at weigh 1.2kg as for my baby boy head down weigh 1kg. I in my 28 weeks. Gynae told both in average weight. But due to umbilical cord, my baby boy doesnt received enough nutrition and blood. Is there any thing I can do to make my twins gain weight? My gynae told too that i could deliver them preterm labor. I not ready to deliver them due to them being small. Or at least till 37weeks.. Mummys, is there a way to prevent preterm labor?