Without the reproductive function no species would be able to survive. This is one of the most fundamental areas dog breeders need to understand, because it has a direct impact on the success of their endeavours. The canine species has many particularities when it comes to reproductive function, which we examine here in simple terms.

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Genital anatomy

Female genital anatomy

© Diffomédia/Royal Canin

The vulva consists of two labia, connected by a ventral commissure (line) and a dorsal commissure (line). The clitoris, which is the analogue organ to the male penis, is relatively small and located in a clitoral fossa on the floor of the vestibule. The vagina, the organ of copulation, is characterised by its great length (15 cm in a medium-sized breed). The uterus, the organ of gestation, consists of a body and two horns in which the embryos are distributed during gestation. At the very top, the ovaries are surrounded by a fat mass. After ovulation, the ova (eggs) move along a narrow tube, the oviduct, where they are fertilised by the male’s spermatozoa. The embryos then move to the uterus, where they attach and grow.

Male genital anatomy

© Diffomédia/Royal Canin

The spermatozoa are produced in the testes. It takes about two months to produce spermatozoa, which then mature in the epididymis, located along each testis. The epididymis is a small multifolded duct, which if unravelled would measure about 10 feet in length. The spermatozoa only become mobile and fertile once they have matured. Testes and epididymes are located in an outgrowth of skin called the scrotum.

At the moment of ejaculation the spermatozoa are mixed with prostate secretions to dilute the semen. Any prostate problems will therefore have major consequences for fertility.

The penis, the organ of copulation, is characterised by a penis bone, which helps keep it stiff but can fracture in the event of trauma or forceful mating. This commonly happens when a mating couple are abruptly separated.

The erectile bulbs at the base of the penis grow hugely during erection, generating a reflex spasm in the vagina during mating, which retains the penis in the vagina.


Puberty in males

The age at which puberty occurs primarily depends on the adult size of the breed (from six months in miniature breeds to 18 months in giant breeds). In males it corresponds to the production of the first fertile spermatozoa. As fertility diminishes at an earlier age in large breeds (a phenomenon probably linked to thyroid ageing), large breeds are fertile for a shorter period of their life. Sperm fertility begins to diminish at around 7 years of age in giant breeds.

Puberty in females

In general, females are around two thirds their final adult body weight when they experience their first oestrous cycle. In small breeds this is around 6-8 months, whereas in some giant breeds it will not occur until 12 or even 20 months.

These are general parameters, however. The first oestrus (heat) can be discreet (little blood discharge, little attraction of males) and may even pass unnoticed. This is called silent heat. It is important to make the distinction between puberty (the ability to ovulate) and nubility (the ability to go to term and whelp). Bearing in mind that the birth canal is not yet fully developed during the first oestrous cycle and maximum fertility is not achieved until second, third or even fourth oestrus, it is inadvisable to mate a female during the first heat. Minimum age limits are in place for pedigree female reproduction in most countries.

During puberty the female reproductive system adopts a cyclic rhythm which is generally expressed as two periods of oestrus every year.

© Diffomédia/Royal Canin

In general, females are around two thirds their final adult body weight when they experience their first oestrous cycle.


Oestrous cycle

The bitch’s sexual cycle

Bitches ovulate once every oestrous cycle. This ovulation is spontaneous, which means it is not triggered by mating as it is in cats, for instance. The oestrous cycle breaks down into four successive stages:

• Pro-oestrus, during which the body prepares for ovulation

• Oestrus, which is the period of ovulation

• Dioestrus or metoestrus, which corresponds to the length of a gestation

• Anoestrus, which is the period of sexual inactivity

© Diffomédia/Royal Canin

The duration of the individual stages can vary. Only dioestrus / metoestrus is relatively stable (60 ± 20 days). Heat is the period comprising pro-oestrus and oestrus, which lasts an average of three weeks, although the actual duration depends on the date of ovulation, which in turn varies from one female to another, and from one cycle to another. Therefore, just because a female ovulates 12 days after the first bloody discharge in an individual cycle, it does not mean that ovulation will occur at the same interval the next time around.

© Diffomédia/Royal Canin

The cycle

© Diffomédia/Royal Canin

During pro-oestrus, under the influence of the pituitary gland, the growing ovarian follicles secrete oestrogens, which are hormones that trigger changes in behaviour (attraction to males, need for affection) and physical changes (discharge from the vulva). The vulva becomes flushed and oozes a bloody discharge, which attracts males, although the bitch will not allow mating at this stage.

Males are generally accepted during oestrus. In this part of the cycle females often adopt a posture in which the tail is carried to the side due to the stimulation of the vulva. This sign should, however, be interpreted with caution in some bitches who will accept males outside their ovulation period. During oestrus, vulval discharges are lighter and become less abundant. The mucus turns thinner, which facilitates the progress of the spermatozoa.

In this phase the ova are still at an immature stage, during which time they are called oocytes. They generally become fertile after 48 hours.

Unlike most species, in dogs the ovaries begin to secrete progesterone a few days before ovulation. The blood progesterone increases gradually, regardless of whether fertilisation has occurred. In dogs, measuring blood progesterone provides information on ovulation but not on pregnancy.

Progesterone is secreted throughout dioestrus by the corpora lutea, which “lay” the oocytes. This hormone prepares the uterus for the implantation of the embryo and enables its development in the event of pregnancy. Its production falls abruptly two months after ovulation, enabling the commencement of lactation and involution of the uterus or the complete quiescence of the female reproductive system (anoestrus).


Fertilisation, which is the union of a spermatozoon and an ovum, occurs in the fallopian tubes. Towards the end of the first week after fertilisation the embryos move from the fallopian tubes to the uterine horns. They float in the uterine fluid and may even move from one horn to the other. The embryos implant in the uterus fairly late, around the 16th day after ovulation. This is when they begin to grow.

The development of the future puppy can be split into two phases:

• Embryogenesis, in which the organs begin to form. At the end of this phase the future puppies are still very small, although most of their structures are in place and all the organs have started to grow. The embryo is now a foetus, which is recognisable as a given species.

• Weight gain: the foetus gains more than three quarters of the puppy’s birth weight after day 40 of gestation.

© Diffomédia/Royal Canin
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