17 Nov Pregnancy
Hormones and pregnancy.
Pregnancy is sustained by the presence of a large quantity of a certain hormone: progesterone. This is a secreted by the corpus luteum of the ovary from the start to the end of the pregnancy. Progesterone is always present during the two months following the heat period whether or not the bitch is pregnant. Therefore we cannot use this hormone to determine whether the bitch is pregnant. However, knowing the level is useful because any sudden drop in the progesterone level in a bitch may terminate the pregnancy. German Shepherds are prone to this type of problem (referred to as luteal insufficiency). It is worth noting that a bitch which has had this problem in the past will not necessary be affected by it during her next pregnancy.
Conversely, if a bitch aborts due to an infection or some trauma etc., the progesterone level will often fall rapidly.
To summarize, assessing the progesterone level during pregnancy diagnosis will it possible to determine whether a bitch is unfertilized or whether she has aborted.
The placenta permits exchanges between mother and the unborn litter from the time of conception until they are born. The placenta forms a circular band around the foetus (the so-called zonary placenta) . Its dark green pigment is the reason for the characteristic colour of the first discharges hen whelping. It is interesting to note that it secrets a hormone, relaxin, which can be used for diagnosing pregnancy.
The unborn puppy is wrapped in several pockets. The first, the amnion, is filled with a amniotic fluid. It acts as a shock-absorber, has a nutrient effect and plays a detoxifying role. The second, which surrounds the amnion, is the allantois. This is the well known “water sac” which is pierced before the bitch whelps.
Confirmation of pregnancy
The fertilization of the ovum by a spermatozoon forms an egg that must undergo several divisions and travel to the uterus before becoming implanted in the uterine mucosa. In bitches, the implantation does not incur until 17 days after fertilization, and results in the formation of embryonic vesicles that cannot be detected by ultrasound until the third week of pregnancy (18 days at the earliest).
After the third week, careful palpation of the abdomen can reveal beading of the uterus, if the bitch is not too fat and the abdominal wall is relaxed. Between the fifth and the sixth weeks, the diameter of the uterus reaches the size of an intestinal loop. During this period, it is difficult to distinguish between a gravid uterus and a loop containing hard stools.
There is no point in taking X-rays until the end of pregnancy, since the foetus’ skeleton does not become calcified (and thus, opaque to X-rays) until after the 45th day.
Other techniques for diagnosing pregnancy occur too late or are too uncertain to be reliable. These include behavioural changes, detection of foetal heartbeats (audible in some bitches during the last two weeks of pregnancy) by auscultation, changes in the blood (sedimentation rate, hematocrit), or mammary development.
Currently the earliest diagnosis of pregnancy is provided by ultrasound.
There is also a blood test based on the production of relaxin by the placenta which is thus only present during pregnancy. This hormone begins to b noticeable towards the twentieth day, but it is preferable to wait until the twenty fifth day to increase the reliability of the test. This test can be carried out as early as ultrasound, but does not give any indication of the size of the litter or of the stage of pregnancy reached.
The stages of pregnancy.
Gestation in dogs lasts from 58 to 68 days (63 days on average). The differences observed among bitches are linked to the difference between the mating date and the actual date fertilization.
In fact, spermatozoa can live for up to five days in the female genital tract until the ova become fertile.
After fertilization, the eggs become embryos, which travel from the oviducts to the uterus and space themselves out through the two uterus horns. Implantation of the embryo in the uterine mucosa does not occur until sometime between the 17th and 19th days after fertilization, so confirmation of pregnancy by ultrasound is impossible before that time.
Transformation of the embryo into a foetus, and subsequent foetal growth, are possible thanks to nutrients brought by the placenta and to the development of other membranes (the amnion and the allantois) that surround and protect the foetus.
Foetal growth does not become externally visible until the second half of pregnancy.