What's the earliest you can sex guppy fry? We had a female die, apparently after giving birth (we didn't know she was pregnant when we bought her and she died within a week of us having her) so we caught the 5 babies we could find and put them in a 5 gallon. They're almost 2 weeks I'll now. I keep a close eye on them and like watching them so I notice the little changes in them. Out of 5 only 1 has a little black spot starting to form where her gravid spot would be as an adult. The 4 others that don't have this also have more black starting to show in their tails. Would it be safe to say that 4 are male and 1 is female?