First, I don't sell Serta BUT here are some guidelines that are pretty universal. 1) Did you purchase the matching box springs or just the mattress? If you reused a old box springs that is Serta's way out so to speak. Simply put the wear on an old box springs will transfer quickly to the mattress. 2) How much sag is in the mattress? Run something straight (string stretched tight, long board, etc) from head to foot over the sagging area to see how much gap or sag is there. Most manufactures don't consider it a warranty issue until the gap is 2" or more. The plusher the mattress the more it will compress. This "compression" MAY be what you're seeing. 3) Is the "sag" just a cosmetic problem or is it something you can feel. A certain amount or of compression is normal but too much is a warranty issue. Contact the store where you purchased it or Serta and ask about the length of full warranty. At less than a year old I very much hope you're still under full warranty. A good quality mattress should have a FULL (not prorated) 10 year warranty. I didn't really answer your question but I hope this background information will help you (if not today then maybe in the future). Sweet dreams!

First, I don't sell Serta BUT here are some guidelines that are pretty universal. 1) Did you purchase the matching box springs or just the mattress? If you reused a old box springs that is Serta's way out so to speak. Simply put the wear on an old box springs will transfer quickly to the mattress. 2) How much sag is in the mattress? Run something straight (string stretched tight, long board, etc) from head to foot over the sagging area to see how much gap or sag is there. Most manufactures don't consider it a warranty issue until the gap is 2" or more. The plusher the mattress the more it will compress. This "compression" MAY be what you're seeing. 3) Is the "sag" just a cosmetic problem or is it something you can feel. A certain amount or compression is normal but too much is a warranty issue. Contact the store where you purchased it or Serta and ask about the length of full warranty. At less than a year old I very much hope you're still under full warranty. A good quality mattress should have a FULL (not prorated) 10 year warranty. I didn't really answer your question but I hope this background information will help you (if not today then maybe in the future). Sweet dreams!