Entirely depends on the conditions and the demographics.
Some slave populations are not self-sustaining. The Atlantic slave-system certainly wasn't: if you have a situation where the slaves are valued for their hard physical work then the slavers will often buy males and not worry too much about a balance of sexes. Sure having your female slaves breed sounds good but consider it:
You are taking them out of the workforce while they are pregent and nursing and won't recieve any return on that investment for at least 16 years.
It's easier to buy fresh of the boat; so that's what they did.
It's only the period between the shutting down of the slave trade post 1807 and the abolition of slavery itself that you see real efforts made to encourage captive breeding because the economics then made sense.
As a general point:
It's a lot easier for slaves to raise a family if they aren't being randomly killed, harassed or tortured and their relationships aren't being broken up. So the more humane the conditions - the more likely the population is to sustain itself.