It depends on what moon phase you were born in. The moon you were born in would be when you are more fertile.
For example if you were born under a new moon you would be really fertile around a new moon. When a full moon is present that would bring on af.
So if your born under a full moon then your days for conceiving under a girl sign would be 17-19th, 21st-23rd and 27th-28th .
Hope that might explain it a little more! If you have any more questions about it then feel free to ask.
Good luck and sending you lots of