Season and Ovulation

These two things are very important when it comes to knowing the best time to mate your bitch.  Ovulation is identified by the levels of progesterone in the blood (there are other ways as well but this is the most reliable). To understand how progesterone testing provides you the answers, I'll first explain a little about the reproductive cycle and the hormones involved.

The normal reproductive (oestrous) cycle of a bitch is comprised of 4 stages (although only the first two are when the bitch is "in heat")

1. Pro-oestrus (average 9 days but can range from 2 to 27). This is the beginning of heat. The vulva is swollen and there is a blood-stained discharge. Although the bitch is (very:) attractive to dogs, she will not allow them to mate her
2. Oestrus (average 9 days but can range from 3 to 21 days). This stage is when the bitch will accept the male. The vulva is very enlarged. The discharge is straw-coloured rather than blood-stained. Ovulation occurs spontaneously usually about 2 days after the start of oestrus
3. Diestrus (average 2 months). Approximately 6 days after ovulation the diestrus stage starts. This stage ends just prior to whelping in the pregnant bitch or approximately 2 months after ovulation in the non-pregnant bitch.
4. Anestrus (average 4 months). Duration of anestrus is quite variable among bitches and may be governed by both genetic and environmental variables.

The physiology of the oestrus cycle can be summarised by

- The hypothalamus (part of the brain) causes the production of "follicle stimulating hormone" (FSH)
- FSH controls the development of the ovarian follicles, which in turn secretes the sex hormone oestrogen
- Low levels of oestrogen simulate further FSH to be released and therefore rising levels of oestrogen
- The above process continues until the follicles are mature and about to rupture at which points the high levels of oestrogen start to inhibit FSH but also trigger the release of "luteinizing hormone" (LH)
- It is the release of LH that causes ovulation and initiates the development of the ruptured follicle into a solid body, the corpus luteum
- The corpora lutea secrete progesterone
- High levels of progesterone inhibits the production of FSH and LH
- It is the balance of the two sex hormones oestrogen and progesterone that ultimately controls the oestrus cycle

In my next post I will go in to more detail about the level of progesterone at the various stages and what it tells you about ovulation and the ideal time to mate