@crystabel
The wishes of the person NOT wanting a child come first sorry OP.
The feeling will pass. Many women have it when they are getting to an age where their fertility is getting very low, and they know they have not got long left (to get pregnant/bear children.) It happened to me (at around 42,) and me and DH tried for about a year. Nothing.
Then the feeling passed, and we started using protection again. I am bloody glad, because now, our kids have flown the nest, and we have the house to ourselves, and no ties or responsibilities or commitments - except the bills and the cats and our jobs LOL!
If we had had another baby at that time - when we were in our early to mid 40s (when the youngest of ours was in their mid teens,) we would be tied down now (at around 50,) with a 5 y.o, with all the drudgery ahead of us; of the school run, school gate mafia, having to go to every single doctor, hospital, dentist, medical appointment etc, and all the school stuff like concerts, and swimming lessons, and sitting around for endless hours while they do the sport or activity that they chose, week in, week out!!
Also, driving them around everywhere, having to tolerate other school mums, and other peoples kids eating us out of house and home, and making noise, and not being able to do a damn thing without a little child in tow. And that's not to mention the astronomical cost of having another child!!! Probably struggle to keep friends too, as very few middle aged people want screaming and demanding toddlers/infants around!!!
I mean we both loved all the bringing up kids stuff at the time (some of it!) and we adore our kids, and if we could go back 25 years, we wouldn't change a thing, and would still have them, but we would not want to go through it again.
Also you will be virtually a pensioner, when they're a teen. And possibly retired when they will need financial support to get them through university too. This is something that needs to be thought about too.
Also, there are many risks and lots of other reasons to not have a baby in your early to mid 40s (or older,) that some posters have already listed. And quite possibly, by the age of 50 (when the child is still small/an infant) you will have elderly and ailing parents. Quite possibly a job too. On top of a baby in your mid 40s too!
You will get LOADS of posters coming on here later, telling you about how they - and every woman they know - had a baby at 44 to 52 years old, and everything went beautifully, the baby is fine, they have the energy of a 21 year old, and their rich, six-figure-a-year husband adores them AND the baby. They will tell you about how they may return to their £90K a year job in media/law/the medical profession, or they may try for another baby.
Weirdly, I have only ever known 4 women in my lifetime who had a baby in their mid 40s; one at 42, one at 43, and one at 45, and one at 46. (The 45 y.o AND the 46 y.o. who had a child in their mid 40s BOTH had babies with a lot of medical issues and problems.)
Funny how so many dozens of women pop up on mumsnet who have babies in their mid to late 40s, and there are never any problems ever! (Weird.) 
Don't do it!!!
p.s The irony of the poster above me waving the flag for having 3 kids in your 40s, when her username is @Foreverexhausted PMSL! 