The average gestation period is 63 days from the day of mating (variable to plus or minus one week).

Fertilisation between eggs and sperm takes place in the oviduct (end of the uterus) and the embryos enter the uterine horns around day 11. The embryos then bathe in “uterine milk” until they are attached to the wall of the uterus, via the placenta, at around day 16: implantation.

The embryos then gradually develop in the amniotic sac, connected to the mother by the umbilical cord.
Fetal growth is greatest in the last third of gestation, between 45 and 60 days.

Progesterone, secreted by the corpus luteum (the name given to the ovarian follicle after ovulation) in the ovaries, ensures that gestation is maintained. Other hormones are produced by the placenta, including relaxin, which is essential for the relaxation of peripubic tissue prior to delivery.

The puppies’ skeletons begin to ossify around the 42nd day of gestation, with clear ossification from the 50th day.

The benefits of carrying out a gestation diagnosis

– Know early on whether a bitch is pregnant or not. The diagnosis of an “empty” bitch is different if she aborts early or if she is not pregnant at all.

– Estimated number of puppies to be born. Used to adapt the diet at the end of gestation, to assess the risk of difficult whelping (large number of puppies or, conversely, the risk of single-puppy syndrome (the puppy continues to grow and becomes too large in relation to the supply of oxygen and nutrients provided by the placenta, leading to death in utero).

– Evaluate the puppies’ growth, the presence of resorption (early abortion) or a development anomaly (disproportion between the embryonic ampullae).

Diagnostic methods:

External changes to the bitch:

– Enlargement of the abdomen from the 4th week onwards
– Can be accompanied by abdominal ptosis in the second month, if the litter is large.
– Udder development: from 25 to 30 days
The udders are hard, turgid and congested. The mammary cord is clearly visible. This sign is unreliable because it is also present in non-pregnant bitches (pseudopregnancy).
– Scanty serous and/or whitish vulval discharge may appear around 25-30 days: this is rare but does not indicate the existence of a pyometra, and when it does, is almost pathognomonic of gestation.

Palpation

– Palpation should be carried out as gently as possible, as it can lead to abortion if it is done too frequently and too gently.
– The embryos form a series of oval blisters in the gravid uterus, the most caudal of which can be palpated through the abdominal wall from around the 21st day.
– After 35 days, the foetal blisters can no longer be identified by palpation as they increase in size, merge and can no longer be distinguished. You can’t tell the difference from bowel movements in the colon.
– From 45 days onwards, palpation of the bony relief once again reveals the foetuses.

Auscultation

– At the end of gestation, foetuses have a heart rate of 220 to 240 beats per minute, before which it is difficult to detect them.
– Auscultation is particularly useful at the time of whelping, during a post-term for example, to determine whether the foetuses are still alive (see specific sheet in “medical techniques”).
– Foetal hearts are often difficult to hear in large bitches.

Note: there are doppler devices (small boxes) which can be used to check for the presence of heartbeats. The main error is the confusion between a foetal heart and the sound produced by the bitch’s aorta.

Abdominal ultrasound

Ultrasound is the test of choice:
– Early diagnosis
– Estimation of vitality and early diagnosis of an abortion: observation of the heartbeat, identification of embryonic resorptions and placental disenfranchisement.
– Estimated number of puppies
– Evaluation of puppy growth
It is recommended that the test is carried out between 21 and 25 days after ovulation or 20 to 23 days after mating.
If no heat monitoring has been carried out, it is preferable to wait until 25 days post-sire, in case the bitch was bred too early.

Relaxin hormone assay

In the bitch, relaxin is the most specific gestation hormone yet found. It is only secreted in pregnant bitches.
It is initially secreted by the placenta, from 3 weeks of gestation, and allows the smooth muscle fibres of the uterus to relax. Signal strength is independent of range size.
There are two blood tests for measuring relaxin.
These tests can be used from the 3rd week of gestation, but if the test is carried out too early, there is a risk of a false negative. It is therefore advisable to wait 25 days. If the result is negative, it is advisable to repeat the test a week later.
The test can be used to determine whether a bitch is pregnant, but does not provide any information about a possible abortion (the test will remain positive) or the number of puppies.

Progesterone is always high, so it cannot be used to determine whether the bitch is pregnant or not.

Abdominal radiography

X-rays are a late examination which is not useful for diagnosing pregnancy, but which does allow the number of puppies to be determined accurately.
On the other hand, contrary to popular belief, X-rays cannot tell you whether a puppy will be able to pass through the birth canal, except in the case of a monstrous foetus (major growth anomaly).
– From 45 days:
There is mineralisation of the foetal skeleton, which then becomes visible on the X-ray.
In practice, X-rays are taken in the last week of gestation so that the skeletons are clearly ossified.

References: Centrale Canine – Article written by Dr Xavier LEVY and Dr Philippe MIMOUNI from the Centre de Reproduction des Carnivores du Sud-ouest (CRECS).