The literature that I’m reading provides me with conflicting information. My book on cat health problems (which is the best book on the market1) tells me that the best time to spray a female cat is at 5 to 7 months of age before she goes into her first heat. This implies that female domestic cats can become pregnant as five months of age or just thereafter.
A website on the Internet, spayfirst.org, confidently tells me that cats can become pregnant at four months of age and have a litter at six months of age.
Another website, animals24/7.com, has a page about spaying female cats written by a veterinarian, Dr. Philip Bushby DVM. There was a convention in January 2016 on the age of spaying and neutering. They were challenging the recommendation that the spaying and neutering of cats be delayed until cats are six months of age or older.
They concluded that female cats can become pregnant as early as 4 1/2 to 5 months of age. Therefore in order to minimize the possibility that a female cat can produce unwanted kittens it was decided that this should be spayed by five months of age.
Is also said that many veterinarians and animal shelters spay and neuter kittens as young as seven weeks of age to combat cat overpopulation.
My conclusion is that the answer to the question in the title is that female cats can become pregnant at four to four and half months of age.
Deciding when female cats can become pregnant is absolutely critical in fact in the mission to minimize the creation of unwanted cats. It is said that the simple change of spaying female cats at six months of age to between four and five months of age could reduce shelter intakes enough to balance the number of adopters with cats available. In other words some people think that this simple change would end the overpopulation of domestic cats in America.
I welcome the input of others in comments on these topics.
1 — CAT OWNER’S HOME VETERINARY HANDBOOK