All of a sudden your female will look like she ate a golf ball! Gestation is 23 days after copulation, so you normally can tell for sure about a week before they pop. Their estrus cycle is every four days, so even if he misses out it won't take much longer before they have a second chance.
Do you know how old they are? Sometimes they take a little bit before they start cycling. They have the potiential to breed as early as 6 weeks (this is bad though) but I find they usually don't start until about 2-3 months of age.