This is sad. Are you much younger than your husband? You say "our late 30s or early 40s." After about age 35, it's harder for women to get pregnant, and it's really tough after 40. Also, you say he may not want children at all.
I think you should figure out what you want. You use the word "ruin" when it comes to children, so it sounds like it's the wrong time for you now. In that case, maybe what you want is an abortion.
Adoption is really, really hard. You have to deal with all the people who ask you questions about your pregnancy for about the four months that happen from when you show to when you give birth. It's also hard to know that your child is in the world, being raised by someone else. And it's hard to talk about your first child to your future children: they feel that if you can give away one child you might give them away. Only about 4% of women have the courage to give up a child for adoption.
Finances are tough when it comes to raising a child. In America, it costs about $100 a week to raise a child. If you want to work, then child care for an infant costs $1,100 a month. Would your savings cover that? Do you have maternity care with your health insurance? If you can't work for months, can you live on his income alone? These are all things to consider.
I'd be very careful about having a child with a man who doesn't think he wants children and doesn't want them now.
As to how you tell him, you just say it and deal with it.