I got mine in my 7th month of pregnancy. You’re able to use in during pregnancy and for a year after. I’d contact your midwife soon as possible and get it. It helps with dentist aswell! Any basic check ups or work you need done is either free or cheaper with the exemption letter
I got mine at my first midwife appointment and it lasts until a year after baby so you're still entitled to it, ask GP or midwife/health visitor
Contact your midwife you had during your pregnancy or I was told your gp should be able to give you one, I got mine from my midwife during a home visit after birth