'16 and Pregnant' Star's New Boobs Are The Max Limit

"16 and Pregnant" star Lindsey Harrison is now 21 and STACKED after getting the biggest breast implants available in the US.

Lindsey, who appeared on season 4 of the show, got the boob job done by Dr. Michael Salzhauer, aka Dr. Miami -- the same doc who transformed "Teen Mom 3" star Briana DeJesus' top and bottom.

Lindsey was an A cup before the doc put in 800cc silicone implants ... the maximum silicone implant available stateside. Doctors believe risk factors outweigh the benefits with anything bigger than 800cc.

As for her new cup size -- Lindsey's DDelighted with her alterations.