As a doctor, I am well aware that toxicity within the body can be the main reason that someone struggles with infertility. One of the therapies I do is called Preconception Care, which is detoxification of mother and father prior to actual conception, thus improving the health of mom and dad and increasing chances of not only fertility, but also a healthy pregnancy, a healthy child, and the child's immune system over their lifetime.
Pray that the dad is willing to do this. Men are usually the most resistant to this process. Even if everything about his sperm is perceived as normal, he should be healthy prior to impregnating someone too.
In addition to taking certain herbs and/or certain nutrients, detoxification usually includes removing substances from a person's consumption (via eating, drinking, inhaling, intravenous, applying to the skin or hair, etc.). This removal part can be difficult, because it involves changing your diet. But it can be harder for those truly addicted to a substance.
Does this mean just because something is hard or that you have failed at giving something up over and over again, that you proceed with creating a baby with a higher likelihood of having health problems? NO. But the reality is that this is happening every day. Then we wonder why our children are sicker and more overweight at earlier ages (toxins accumulate generation after generation). Keep in mind that the habits you have will also influence the child even more once they are born because 1) they will see them occurring and on some level probably find it okay/normal, and be more likely to start doing them too, and 2) how you live and behave around them will be altered from how you would if those habits were not existing, and thus it will shape their mindset, emotions, decisions, and how they live their life.
If you want a child that much, love them enough to put in the hard hard work- yes I said 'hard' twice because it may be one of the hardest things you have ever done - to remove addiction and bad habits from your life prior to conceiving a child (6 mo - 1 yr prior), and continue to keep them out while pregnant and breastfeeding. It is up to you if you resume once the child is weaned from breastfeeding (by then, hopefully you will have chosen to stay away permanently), but at least you can give your child better health.
Think about that. Think about how your life could have been different if your mother and father took the time and great effort to improve their health (physical, emotional, and mental) prior to having you, and how that could have changed your life for the better. Be selfless enough to do that for your children - that's love.