Breeding is something we all become involved with at some point in our veterinary career. Therefore, it’s important to understand the basic physiology of the oestrus cycle and foetal development, so you are prepared for any complications or abnormalities.
After spending many years working in a breeding facility as a veterinary nurse, the author gained vital experience in mating, whelping and neonatal care – something she is very passionate about. The author strongly believes we will never stop people breeding, even though there are plenty of puppies around, so we need to try to educate our breeders as best we can. Therefore, we need to fully understand the breeding process ourselves.
Stages of the oestrus cycle
The four stages of a bitch’s oestrus cycle are proestrus, oestrus, metoestrus and anoestrus. The oestrus stage is when a bitch will accept a male for mating and is often called being “in season” or “on heat”.
It is also important, however, to understand the timings of and what happens in each stage of the oestrus cycle so we can advise owners who may be considering breeding on appropriate timings, diagnose abnormalities and recognise potential health issues related to the oestrus cycle.
Proestrus
Proestrus lasts for approximately seven days, and changes seen during this stage include:
- An increase in plasma concentrations of oestrogen
- Swelling of the vulva
- A thin serosanguinous vaginal discharge
During this stage, male dogs are attracted to the bitch, but she will not yet accept a dog to mate.
Oestrus
This stage lasts approximately seven days and is the only time the bitch will allow a male to mate and tie with her.
A surge of follicle stimulating and luteinising hormones cause ovulation, which occurs around two days after the start of oestrus. Progesterone is produced just before ovulation.
Behavioural changes are seen, with the bitch deviating her tail to show her vulva and demonstrating standing behaviour for male dogs. Here the vulva becomes less swollen, and vaginal discharge becomes clearer and more straw-coloured.
Metoestrus
Metoestrus lasts for approximately 55 days. During this stage, the corpus luteum continues to produce progesterone, which will maintain the pregnancy if the bitch has been mated and is pregnant. Birth occurs when the progesterone secretions finish. In a non-pregnant bitch, the progesterone secretions continue for approximately the same amount of time and then progress to the next stage of the cycle.
At the end of metoestrus, prolactin is released. It is produced and released in pregnant and non-pregnant bitches and causes mammary development and preparation for lactation.
It is during the metoestrus phase that pseudopregnancy occurs in a non-pregnant bitch.
Anoestrus
Anoestrus is a phase of inactivity which lasts, on average, four months. In a non-pregnant bitch, levels of progesterone slowly decrease, whereas progesterone levels reduce suddenly at parturition in pregnant bitches. There are no external signs during this phase.
During late anoestrus, follicle stimulating and luteinising hormones are released, initiating the growth of follicles in the ovaries and stimulating oestrogen production from ovarian follicles. This begins the whole cycle again.
Pregnancy and foetal development
The ovum
The ovum is surrounded by a double protective layer composed of the zona pellucida, the innermost layer, and the corona radiata, the outer layer.
Fertilisation takes place in the oviduct, where a sperm burrows into the ova. It penetrates the corona radiata but burrows through with a different type of motility than normal movement. It is during the burrowing action that a change occurs in the head of the sperm, and it releases an enzyme that digests the zona pellucida allowing the sperm to fertilise the ovum.
Once fertilised, the ovum is known as a zygote and continues to travel down to the uterus along the uterine tube.
The foetus
The zygote develops a cavity with a mass of cells towards one end – the inner cell mass from which the embryo will develop.
Then three germ cell layers that will eventually become recognisable parts of the body are formed:
- Ectoderm – the outer layer that forms the skin and nervous system
- Mesoderm – the middle layer that forms the internal organs and musculoskeletal structures
- Endoderm – the inner layer that forms the lining of the gastrointestinal tract and other visceral structures
The yolk sac and trophoblast then form the extra-embryonic membranes, which will become the placenta and membranes covering the foetus at birth.
Once the zygote has travelled down the uterine tube, it invades the hypertrophied endometrium of the uterus to lie within it and become attached to the uterine wall. This is known as implantation, which occurs around 14 to 20 days post-ovulation in dogs.
The extra-embryonic membranes are made up of the yolk sac, chorion, amnion and allantois. These all play their part in the development of the foetus.
As the embryo begins to curl up on itself, the top of the yolk sac becomes narrow to form the primitive gut tube. From this, another diverticulum called the allantois forms (otherwise known as the allantoic sac), which receives urine from the foetal kidneys. As the allantois develops, the trophoblast expands, encircling the embryo to form a double layer of membrane called the chorion and amnion.
As things grow and expand, the allantoic membrane fuses with the chorion above and the amnion below it. This process forms the chorioallantois – an outer membrane that acts as the water bag that ruptures as the foetus moves through the uterus during parturition. The allanto-amnion, the inner membrane, often known as the “slime” bag, is also formed during this process. During parturition, the bag releases lubrication to ease the foetus’s passage.
| Age | Foetal development | Embryo length |
|---|---|---|
| Three weeks | Amnion and allantois are formed | 5mm |
| Four weeks | The limb buds are visible | 20mm |
| Five weeks | The eyelids and ear pinnae are visible | 35mm |
| Six weeks | The digits and external genitalia are visible | 60mm |
| Seven weeks | The colour markings and hair develop | 100mm |
| Eight weeks | The pads of the feet develop | 150mm |
| Nine weeks | Parturition occurs with ears and eyes closed |
The stages of parturition
There are five stages of parturition, and it is important to recognise the markers of each stage so you know what is expected and what is abnormal.
Stage of preparation is when the plasma progesterone levels decrease, and relaxation of the vaginal and perineal tissues begins.
This is also when prepartum hypothermia is seen – something many breeders rely on and recommend to others. Prepartum hypothermia occurs 24 to 36 hours before parturition as the bitch’s body temperature drops due to the sudden lowering of plasma progesterone. During all the author’s time working with pregnant bitches, they have found this fairly hit and miss, with some bitches demonstrating prepartum hypothermia and some not!
One thing the author normally says to owners is that the bitch “drops down” at this stage. By this, the author refers to the pregnant belly looking like it has gone from being a rounded belly to looking like the belly has dropped towards the bitch’s knees.
The first stage commences with the onset of uterine contractions and lasts around 12 hours. There is milk present during this stage, and the dam becomes restless, pants and nests. Shivering and vomiting can be normal during this time due to the initial discomfort from the start of contractions.
Second stage sees contractions increase as the puppies move towards the cervix and into the vagina. The amnion is seen at the vulva during this stage, preparing to rupture on delivery. If the bag is still in place when the puppy is delivered, the dam must break it. If she fails to do this, the person present must break it. The placenta should follow the puppy, normally within 20 minutes maximum.
Third stage is actually the passing of the placenta in humans; however, as the placenta is passed in the second stage between foetuses in animals, it does not always get classed as a separate stage.
Puerperium is the final stage, which lasts for approximately four to six weeks. This is when the reproductive tract returns to normal with uterine involution. Mucoid vaginal discharge is often present during this time, especially during the first few weeks.
Dystocia
In addition to whelping, neonatal care and puppy rearing is another significant subject for consideration. When you think about parturition, however, it is important to understand dystocia as it is commonly seen in veterinary practice. Veterinary nurses should consider:
- Primary uterine inertia – this occurs when parturition does not commence. It is commonly seen in young bitches. The actual cause of primary uterine inertia is unknown, but poor uterine muscles, an overstretched uterus and low blood plasma calcium are suspected factors
- Secondary uterine inertia – this occurs when uterine contractions begin but then stop. This is usually due to uterine exhaustion, which can be due to an obstruction such as a large puppy blocking the way or prolonged time of whelping causing exhaustion and low blood calcium
- Obstruction of birth canal – this is usually an anatomical problem with the bitch rather than an obstruction caused by a puppy
