[header] How to know if a female dog is fertile [title] Look for swelling of the vulva. [step] When the dog is reaching her fertile phase, known as estrus or heat, her vulva will begin to swell. This can be seen on the outside of her body if you look at the area directly below her anus. 
[substeps] The vulva will be puffy and will appear red in color. By the time she is in full heat, the vulva will swell to about three times its normal size.