Cherral Mitchell, 31, needed two midwives to haul out big Alpha before he left staff shocked when he weighed in at more than a stone.
He's since been nicknamed 'Baby Hippo' and 'Butter Bean' due to his birth weight which was a whopping 14lb 15oz.
And the big little one was also a surprise - born despite Cherral getting the contraceptive coil before he was conceived with electrical engineer husband Tyson, 35.
Cherral, who has three more kids, said: "We didn't think he was going to be that big. Everyone kept laughing when his head came out. My husband Tyson was like 'oh my God he's chunky'.
"There were two nurses pulling - one was trying to push him down to get him out. The nurses said it had to be the biggest baby and were on their phones Googling.
"He's a bit of a pumpkin baby with it being so close to Halloween."
Alpha, who was born at 38 weeks on Thursday, is believed to be the UK's third-biggest baby with the current record holder being 15lb 8oz Guy Carr, born in 1992.
The second biggest is 15lb 7oz George King, who was born in Gloucester in 2013.
Doctors have put Alpha's weight down to a sweet tooth as they believe he was snacking on Cherral's sugary fluids while in the womb.
Cherral, from Thame, Oxon, said: "They think that because I was diagnosed quite late with gestational diabetes, Alpha was in amniotic fluid that was sweeter.
"So he was drinking a lot and peeing a lot and getting a high intake of sugar."
Alpha was estimated to be 11lb 1oz in a scan at 37 weeks, but he ballooned by three pounds before he was born a week later through a C-section at John Radcliffe Hospital in Oxford.
Cherral, a stay-at-home mum, said: "My stomach was big but not that massive so I don't know where he was hiding.
"I keep saying I gave birth to a baby butter bean. My uncle Jason calls him a baby hippo."
Cherral already has a son - Lyon, three - and daughters Rogue-Angel, four, and Twyla Bay, 10 months - but said that Alpha is the biggest by a long shot.
She said: "My other children were all normal."
Alpha is currently in NICU where he's being monitored by neonatal nurses but is 'on the mend'.