Is there a way to help baby gain weight?

Rachel • Di/di twin boys and one on the way👦🏻👦🏼👶🏼

My twins are on the smaller end, one is in the 38th percentile and the other is in the 21st percentile. The doctor said he’s not concerned unless they’re falling towards the 10th percentile. I’m wanting to help my babies pack on some fat before they’re born in 4-6 weeks. Scared they’ll be in NICU for a long time:(