Please choose the correct ending to complete the given sentence: Pets and Animals: [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.

Ending1: Her vulva will swell if she's mixing up her diet. Place a clear, hand-held thermometer at the vaginal opening to check if she's pregnant. Ending2: [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. Ending3: It can also appear when a dog is approaching a foal, baby or male dog. [substeps] Uterine secretions during its day during estrus can appear during estrus. Ending4: [substeps] See if the vulva is swollen at any point after intercourse. You should also check if the dog is " breeding ".

Answer format: ending1/ending2/ending3/ending4