ABSTRACT: In order for a female animal to reproduce her species, she must first advertise her willingness to mate to the male animal, conceive and finally deliver healthy offspring. All of these processes involve the coordination of behaviour patterns and physical changes within the ovary and the reproductive tract which are controlled by the reproductive hormones. This article describes the oestrous cycle of the bitch, the process of ovulation and the complex controlling inter¬relationship between the four hormones secreted by the ovaries and by the pituitary glands.


In a recent article, we studied the anatomy of the reproductive tract of the bitch (Canis familiaris) and now it is time to understand the complex physiology involved in reproduction. When describing this physiology it is important to realise that female mammals have much in common but the aim of this article is to focus particularly on the bitch.

The most significant part of the female tract is the ovary, the function of which is (i) to produce ova that, when fertilised, will become the embryos, and (ii) to secrete the reproductive hormones oestrogen and progesterone which control the oestrous cycle. The remains of the tract undergo hormonally-induced changes in preparation for the implantation of the embryos and for their development into viable foetuses.

Oestrous cycle

This may be defined as ‘the cycle of events that occur in all post-pubertal non-pregnant female animals involving regular limited periods of sexual receptivity’. It involves simultaneous events in several parts of the reproductive tract, controlled by a series of complex hormonal pathways. These events (Table 1) occur in:

1.   The ovary – see ovulation

2.   The endocrine system – see hormonal control

3.   The uterine horns and uterine body thicken and become more glandular

4.   Behaviour of the animal that ‘advertises’ to the male that the female is in a sexually receptive state.

The pattern and signs of the cycle vary according to the species in terms of the individual phases and their lengths – the oestrous cycles of the bitch and the queen are quite different.

The average bitch reaches sexual maturity at about six to nine months of age, although this varies with the size and breed of the dog – larger breeds mature much later. Sexual maturity is marked by the onset of the first oestrous cycle.

Bitches commonly have an oestrous cycle every six months and are then described as being ‘in season’. The bitch is said to be monoestrous which means that during each period of ovarian activity there is one period when she will accept the male – this is known as oestrus. There is no recognised breeding season – bitches may come into ‘season’ at any time of year.

The bitch is described as being a spontaneous ovulator – she will usually ovulate on day 10 of the complete cycle whether or not she is mated. Other spontaneous ovulators include the horse, sheep and human being.

The stages of the cycle related to the bitch’s behaviour and to changes within the ovary and reproductive tract are summarised in Table 1 and illustrated in Figure 1.

Figure 1: Phases of the oestrous cycle of the bitch (Reproduced with permission from Aspinall, V. & Cappetlo, M. (2009) Introduction to Veterinary Anatomy and Physiology. Butterworth Heinemann, Oxford p132)

The total length of time that a bitch shows overt signs of her season is about four weeks, but within the ovary there will be histological evidence, viz. a visible corpus luteum for about another three months.

In the period between cycles there is a long, and sometimes variable, anoestrus during which there is no discernable ovarian activity. Towards the end, new follicles begin to develop in the ovary and secrete oestrogen. When blood levels have risen sufficiently they will initiate the external signs of pro-oestrous and the cycle begins again (Figure 1).


This is the process of releasing the mature ova from the ovary, which consists of a framework of connective tissue, blood capillaries and germ cells. At birth the ovary contains all the germ cells that will be needed during the animal’s reproductive life and these act as a reservoir from which the follicles develop. Many of these germ cells will eventually atrophy and disappear.

At puberty, at the beginning of the first oestrous cycle, some of the germ cells begin to develop into primary follicles. In litter-bearing or multiparous animals, such as the dog, there will be many follicles in each ovary.

As the cycle continues, the follicles mature into Graafian follicles, each of which consists of an outer double layer of cells surrounding a fluid-filled cavity which contains an ovum whose nucleus contains the haploid (half the normal number) of chromosomes. The Graafian follicles secrete oestrogen.

Towards the end of pro-oestrus – and at the start of oestrus – the follicles reach full size and rupture releasing the ova, which pass down the oviduct where they may or may not be fertilised by sperm from the male. Ovulation usually occurs around day 10 of the cycle (Table 1) and is a result of rising levels of luteinizing hormone (LH) (Figure 2).

Figure 2: Graph shows the relationship between the levels of reproductive hormones during the oestrous cycle of the bitch

After ovulation, the follicular tissue remaining in the ovary goes through several stages:

1.   There may be a little bleeding into the centre of the follicle which produces a transient stage known as the corpus haemorrhagicum.

2.   The blood is resorbed and the follicular tissue then becomes organised to form the corpus luteum. This secretes progesterone and the reproductive tract remains under its influence for the remaining stages of the oestrous cycle. The corpus luteum gradually regresses as new follicles develop at the start of the next oestrous cycle.

3.   When the corpus luteum has regressed, it is replaced by a small white scar known as the corpus albicans. It is possible, when examining the ovary microscopically, to detect how many ova have been released during the individual’s reproductive life.

If the animal is mated and becomes pregnant, the corpus luteum remains in the ovary for almost the entire gestation period and continues to secrete progesterone, which is largely responsible for maintaining the pregnancy.

In the non-pregnant bitch, the corpus luteum of the cycle remains in the ovary for almost as long as it does in pregnancy, so levels of progesterone remain high – this
is the reason why some bitches develop the symptoms of a false pregnancy (also called pseudopregnancy or pseudocyesis). 

Hormonal control

There are several hormones (Table 2) involved in a complex interaction (Figure 3) that brings about the phases of the oestrous cycle (Table 1).


Figure 3: Inter-relationships between the female reproductive hormones. !Reproduced with permission from Aspinall, V. & Cappello, M. (2009]1ntroduction to Veterinary Anatomy and Physiology. Butterworth Heinemann, Oxford. p75]

1.   At puberty or at the start of the breeding season various external stimuli, such as day length, environmental temperature and the presence of the male are perceived by receptors within the eyes, ears and nose and stimulate the hypothalamus within the forebrain.

2.   The hypothalamus secretes gonadotrophin releasing hormone (GRH) and the oestrous cycle begins. GRH is carried by the blood to the anterior pituitary gland which then secretes follicle stimulating hormone (FSH).

3.   FSH stimulates the germ cells within the ovary and some develop into ripe Graafian follicles which secrete oestrogen.

4.   Oestrogen prepares the reproductive tract for coitus by causing the lining to become thickened and more glandular, mucus production increases, the vulva of the bitch enlarges and there is also a blood-stained vaginal discharge. Oestrogen also inhibits further production of FSH and stimulates the secretion of luteinizing hormone (LH) from the anterior pituitary gland.

5.   LH acts on the follicles and some of the more mature cells luteinize and begin to secrete progesterone.

6.   Eventually each follicle ruptures and an ovum is released. The remaining

7.   Falling levels of oestrogen and rising levels of LH and progesterone produce the behavioural signs of early oestrus and make the female stand still and allow mating by the male (Figure 2). 

8. Progesterone prepares the reproductive tract to receive the fertilised ova by causing the uterine walls to become thickened and more glandular, and the mammary glands enlarge in preparation for the production of milk. Progesterone also inhibits the secretion of GRH, thus preventing the formation of more follicles and helping to maintain the pregnant state.

9.   If the animal is not pregnant, the corpora lutea regress, the inhibition of GRH is lifted, and the oestrous cycle begins again.

So to summarize, the oestrous cycle of the bitch results from a complicated inter-relationship between the ovary, the uterine tract and the behaviour of the bitch. These occurrences are brought about and controlled by changes within the endocrine system. What makes it even more difficult to understand – and to describe – is that everything happens at once! 


Victoria Aspinall BVSC MRCVS

Victoria qualified from Bristol University vet school and went into small animal practice. After raising her four children she taught at Hartpury College, where she started the veterinary nursing department. She subsequently founded Abbeydale Vetlink Veterinary Training which is a VNAC in the west of England.

Victoria is an associate lecturer in veterinary nursing at Bridgwater College, Somerset. She has written and edited many books for vet nurses.

To cite this article use either

D01:10.1111/j.2045-0648.2010.00042.x or Veterinary Nursing Journal Vol 26 ppl 53-157


ALLEN. W. EDWARD [19921 Fertility and Obstetrics in the Dog Blackwell Scientific Publications. Oxford. ASPINALL, V. 120061 The Complete Textbook of Veterinary Nursing. Butterworth Heinemann. Oxford. ASPINALL. V. and CAPPELLO. M. 120091 Introduction to Veterinary Anatomy and Physiology 2nd ed. Butterworth Heinemann, Oxford.

LANE. D.. COOPER. B. and TURNER. L 120071 BSAVA Textbook of Veterinary Nursing. 4th edition. BSAVA, Gloucester,

Veterinary Nursing Journal • VOL 26 • May 2011 •