The phrase "regular bathtub" refers to a typical or ordinary bathtub that most people have in their homes. It is not a special or unusual type of bathtub. Full definition
However, it is always a good idea to go for the Best Baby Bath Tub that you can place in the regular bathtub to make bathing more convenient and easier. (welcomeparenthood.com)
And you won't actually be putting your baby in a regular bathtub. (verywellfamily.com)
If you've been using the baby bath somewhere other than the bathroom, move it into the bathroom for a while to familiarize your child with the environment before putting it into the regular bathtub. (verywellfamily.com)