Chrissy Teigen is having Botox to relieve her pregnancy headaches.

The 'Sports Illustrated' model - who has Luna, four, and Miles, two - is expecting her third child with her husband John Legend and she has revealed just why she is planning to have the cosmetic surgery during her pregnancy.

She explained on Twitter: ''I get really really bad pregnancy headaches. Was thrilled to be cleared to do neck muscle botox along with a crazy combo of beta blocker shots and radio wave frequency something something doctor terms. anyhow man it's just so bad but I see the light finally ... Jaw Botox for my grinding was a MUST for my regular headaches. You can also do behind the brow for migraines. Life changing ... Yeah if you have the means to go to a neurologist instead of cosmetic it's a lot better and safer because they talk to your OB. (sic)''

Meanwhile, Chrissy previously revealed she was unknowingly pregnant during her breast implant removal surgery because she'd got a false negative on her test.

Asked if she knew she was pregnant during her surgery in June, Chrissy replied: ''Oh, it's quite a story. lol ... a few weeks after surgery, I took a test. And for many years now, I've taken pregnancy tests almost every month, praying to see a positive one day. Just wishful thinking. I never had a positive before. so the morning of John's album release, he wakes up at 3am to do good morning america. I woke up with him and was like man, should take my monthly test to be disappointed....

''I was not disappointed. But I was scared s*******. Was pretty positive you shouldn't get your boobs out while pregnant? Pretty sure. So we prayed to the boob surgery gods that everything would be okay. Went to every appointment terrified. Even without the surgery, I didn't think I could get pregnant naturally anyway. So the odds just felt...bad. But what they say so often can be true. When you give up on trying, life has a way of surprising you. In summary, my boobs hurt. (sic)''