The reality TV star originally planned to head to Las Vegas for the milestone with his friends, but has decided that he would prefer to stay with his "beautiful" partner.
"I've been saying for absolutely ages that I was planning a massive boys' trip to Las Vegas for my 40th birthday in February, but things have obviously changed a bit since then and I now think I'd rather do something else," he wrote in his New magazine column.
"Why would I not want to spend my 40th with my beautiful girlfriend?
"I'll still do something amazing but Vegas is on hold. Maybe I'll have my stag do there instead... Just kidding!"
The 39-year-old singer has been dating the 23-year-old medical student since July.
Andre also revealed that MacDonagh's family have spent plenty of time with his children Princess, 5, and Junior, 7.
"I took Junior and Princess to stay with Emily's family in Taunton, Somerset, last weekend and we were well and truly spoiled rotten.
"Emily's parents have a lovely big family home – which is lucky given that they have five children! – and it was non-stop fun and games for my kids."
Peter Andre recently hinted that he may apply for university in order to keep up with his partner.