Vancouver Feline Hospital - Cat Veterinary Clinic

CAT NORMAL REPRODUCTIVE CYCLE

Species differences are not mentioned so don't apply this to your dog, rabbit or yourself. The cat is seasonally polyoestrus (comes into heat only at a certain time of the year, and then comes into heat more than once during that season). She responds to increasing daylight length and so her natural breeding season is in the spring and summer months. Our indoor cats and cats living close to the equator where seasonal variations in daylight length are not so marked, may breed throughout the year.

So as daylight length increases during the last days of winter, the queen's (a queen is an entire or non-spayed female cat) system responds by increasing oestrogen and her ovaries start developing follicles. The follicular phase is the stage when she has very large "ripe" follicles on her ovaries and her blood oestrogen concentration is at its peak. The behavioural (heat) oestrus phase is when she calls, rolls, crouches and allows mating. These two phases may be perfectly superimposed or overlapped slightly. When the behavioural phase lasts longer than the follicular phase the queen may be mated to late. Sometimes the follicular phase can reoccur before the end of the behavioural oestrus and it will appear as if she remains on heat for 3 weeks. Heat lasts about 1 week. If the queen is not mated, her oestrogen level drops, progesterone levels rise again, and she starts the next round of "being on heat". This 3-week cycle will continue until she is mated or spayed or the daylight length decreases enough for her to go into winter anoestrus.

It is important to realise that the queen does not ovulate unless she receives adequate stimulation to do so (as in mating). Heat is not an indication of ovulation. The lack of behavioural signs of heat does not mean she has been mated or is pregnant. Progesterone in the queen does not necessarily prevent oestrus behaviour. (High dose progestogens - synthetic progesterone - are used in some countries as birth control - side effects include mammary hyperplasia and uterine infections).

Each time she is mated, her pituitary releases a little pulse of LH. These pulses of LH eventually superimpose and collectively cause a LH surge. Ovulation will happen 1 day after the LH surge. So to ovulate she must mate often (the rule of thumb is 4 times per day for 4 days). Sometimes people advise to stimulate the queen to stop behavioural heat by inducing ovulation (apply the rule of thumb!). Veterinarians can also inject GnRH (gonadotrophin releasing hormone), which will stimulate ovulation. Not all cats mated will ovulate - inadequate mating interval will mean inadequate LH surge, or a too early or too late mating with an LH surge where the follicles cannot respond.

So she is successfully mated, returns home safely and one day after the LH surge she ovulates. Her oestrogen levels will drop and progesterone levels rise. The follicles that ovulated become the Corpus Luteum (CL). The CL produces the progesterone needed to maintain the pregnancy; and the queen is reliant on a functional CL to maintain pregnancy.

If she ovulated but didn't conceive (e.g. GnRH injection), the CL regresses in about 1/ the time it would otherwise, so she goes through a long interoestrus (or dioestrus) period of 5-7 weeks before coming on heat again.

If she did ovulate and conceived, pregnancy lasts exactly 65 days from the LH surge. Towards the end of pregnancy there is a gradual decline in progesterone, until the last 2 days when there is a sudden drop of progesterone. This sudden drop is the stimulation for parturition (giving birth) and the release of the hormone prolactin from the pituitary. Progesterone is the hormone that stimulates mammary gland development and secretion of milk and remains high throughout lactation. Lactation lasts for about 6-10 weeks, and during this time the queen does not cycle - she is in lactational anoestrus. When prolactin levels drop and the kittens are weaning, she starts cycling again - to be ready for the next mating only 1-3 weeks after weaning! She is just a kitten factory - with little or no time to recover physiologically before the next set of reproductive demands on her system.