The same way you had convinced her to have sex with you, but I doubt if she will leave her husband of probably 30yrs, but figuratively she would if you put her at a point and ask her to deicide between you and her husband. After all if she is ready to bear your child she must be so close to you, so ask her to decide and let her know your actual intentions to divorce your existing wife and marry her to take care of her and your baby.
She would resist initially but she would do it depending up on the way you treat her normally when not having sex.
Its about time she got pregnant, has she been on pills all these 6yrs because how come she didn’t get pregnant all this while? This is what sex turned love, classic case this is. Go on, talk to her and seek her permission.
If she is hesitant to do this fearing society, tell her you love her, so no worry in doing this. Your question has kind of disgusted me but it was some kind of consolation, since you mentioned you love her and she does it back, good luck.