My boys are 18 months apart. My oldest was 9 months old when I got pregnant with his brother.
It was tough in some ways, but it was great in others.
Tough: Carrying a toddler who refuses to walk AND an infant in his carrier.
Awesome: Being done with diapers in short order.
Tough: One barely walking, the other wants to run! RUN NOW! :-)
Awesome: They're best friends and very close.
Tough: One's getting molars, the other is cutting first teeth.
Awesome: Hand-me-down toys and clothes, not having to worry that toys aren't age-appropriate (no choking hazard issues), teaching sharing is EASY (I don't really remember teaching it individually, actually).
I can't think of more tough stuff.
More awesome stuff: You're already really good at parenting an infant and toddler. You know how to do better in many ways. Kiddo won't even remember not having a sibling. They'll keep one another busy and entertained (which means that you won't be the primary entertainer). They'll have one another even after you and your husband are gone.
The reasons go on and on. I recommend trying as soon as you can. :-)