DACHSHUND: "Heat"

 

 

Introduction

Many aspects of canine pregnancy are unique among veterinary domestic species. However, an understanding of the time course and clinical correlates of ovulation, fertilization, embryo and fetal development, and pregnancy specific changes in maternal physiology is essential when providing clinical services such as breeding management and monitoring of pregnancy. Such understanding is also important for decision-making in cases of pregnancy failure, elective caesarian section and dystocia. One important fact is that gestation length and events of gestation are very repeatable and predicable when viewed in relation to the time of ovulation or the preceding the luteinizing hormone (LH) surge (Table 1 and Table 2). The interval from the LH surge to parturition is predictably 65+/- 1 days in nearly all canine pregnancies. That is true, despite the fact that the normal interval from breeding to whelping can range from 55 to 70 days.

Table 1. Events and clinical correlates of canine pregnancy through the time of implantation and pregnancy detection, aligned to days from pre ovulatory LH surge.

Days Events and changes in parameters -23 to -3 Onset of proestrus (heat) - average Day -9 -3 to +6 Onset of estrus behavior - average Day 0 to 1 -3 to +8 First acceptance of intromission and mating - average Day 1 -3 First day a single mating has significant fertility. 0 Pre ovulatory LH surge - time of major increase in serum LH 0 Increase in progesterone from levels of 0.3 - 08 ng to levels of 0.9 to 3.0 ng/ml. 0 Onset of peak fertility for single matings by high-fertility studs 2 Ovulation at 38 - 58 h after LH surge 3 Primary oocyte(s) in oviduct. Potential penetration by sperm 4 Oocytes presumably still without polar body or female pronucleus 5 Maturation of oocytes in distal oviduct. Fertilization completed if already bred 6 Bred: 1-2 cell embryo. Non-bred: mature oocytes still fertile 7 Bred: 2 cell embryo. Non-bred: viability of some oocytes declines or lost. 8 Bred: 4 cell embryo. Non-bred: late mating results in small or no litter 9 Bred early: 4-8 cell embryo. Bred later: 4-8 cell embryo. Mating rarely fertile 10 Oviductal embryos: 8-16 cells 11 Oviductal embryos: 16 -32 cell morulae 12 Morulae inside zona pellucida found in uterine horns 13 Intra-uterine migration of blastocysts between horns 14 Migration within uterus continues 15 Ultrasound (U/S) does not detect any difference due to pregnancy 16 Enlargement of embryos and thinning of zona pellucida 17 Blastocyst enlargement continues. Migration stops 18 Zona enclosed blastocyst in > 1 mm diameter uterine vesicle. U/S detectable vesicle 19 Uterine vesicle visible on U/S. Embryo + zona pellucida. Mucoid coat. 20 Embryo expansion in >2 mm x 3-6 mm uterine vesicle. Zona absent. Thin mucoid coat 21 Blastocysts touch, but are still unattached to, endometrium. Cannot be flushed. 22 Uterine swellings grossly visible by d 21-23. Embryo attached. Invasion begins. 23 Placental trophectoderm invasion of endometrium continues. U/S detects embryo mass 24 Heart beats may be visible on U/S. Palpable 1 cm uterine swellings 25 U/S detection of heart beat 26 Rises in serum relaxin and acute phase proteins (fibrinogen) in some bitches. 28 U/S detects zonary placental mass. Relaxin typically detectable

 

Table 2. Events and clinical correlates of canine pregnancy from implantation to parturition, aligned to days from pre ovulatory LH surge

Days Events and changes in parameters 22 Uterine swellings grossly visible by d 21-23. Embryo attached. Invasion begins 23 Placental trophectoderm invasion of endometrium continues. U/S detects embryo mass 24 Heart beats may be visible on U/S. Palpable 1 cm uterine swellings 25 U/S detection of heart beat with high-resolution equipment 26 Rises in serum relaxin and acute phase proteins (fibrinogen) in some bitches 28 U/S detects zonary placental mass. Relaxin typically detectable. Heart beats clear 30 Palpable, distinct 3 cm uterine swellings. Easy palpation. Prolactin increases 32 Increased prolactin levels detectable 34 Maternal anemia typically evident 36 Palpation yields less-distinct uterine masses. U/S detection of fetal limb buds 38 Embryo still shorter than placental girdle 42 Embryo starts to become longer than placental girdle 46 X-ray first detects skull and spine. Obvious increase in mammary development 50 Acute phase protein levels near peak 54 X-ray may detect limbs and pelvis 56 Teeth still not visible on X-ray 58 X-ray readily detects limbs and pelvis; possibly teeth 60 X-ray readily detects teeth by now or next day. Progesterone above 3 ng/ml 62 Progesterone begins to decline. Nesting, restlessness begins over next 2-4 days 63 Early parturition / short gestation, but not abnormal 64 Early parturition / normal gestation. Progesterone below 2 ng/ml 12-24 h pre-partum 65 Mean parturition date. Predicted whelping date 66 Late parturition / normal range 67 Very late parturition, but not abnormal absent signs of dystocia 68 Over-due if normal signs of nesting and whelping are absent

 

Fertilization and Gestation Length

The reason why gestation length in dogs is relatively consistent when measured from the day of ovulation but highly variable when measured from the day of breeding is partly understood. In the vast majority of bitches parturition occurs 64, 65 or 66 days after the ovulatory surge in LH [1]. The latter represents the acute release of LH from the pituitary in late proestrus or early estrus. The LH surge triggers the event of ovulation. Since the day of the LH surge can be measured or estimated with reasonable accuracy, timing events from that day, and using it as the reference point (Day 0) can be helpful. A 64 - 66 day gestation length measured from the LH surge to parturition is the same as a 62, 63 or 64 day interval between ovulation and parturition, since ovulation has been estimated to occur 2 days after the surge in LH [4].
In contrast, using the day of mating as a reference point, as observed in cases of just a single mating or insemination, parturition can occur as early as 56 days later and as late as 68 days later. Similarly, a large variation in apparent gestation length can be encountered when counting from the first of multiple matings or the last of multiple matings, the extremes encountered differing by 2 weeks. For instance, if a bitch is held for an aggressive stud dog and forced to mate starting 3 - 5 days before the LH surge, the interval from first mating to whelping may be as long as 69 - 70 days. And, in rare instances where a bitch is still fertile 9 or 10 days after the LH surge and is bred then, the interval from mating to whelping can be as short as 55 or 56 days.
Part of the explanation is that dog sperm may, in some instances, survive in the bitch's tract for up to 7 or even 9 days and still remain viable in terms of being able to achieve fertilization and result in pregnancy. It is possible that in dogs as in other species, many sperm die or loose fertility after 1 or 2 days. However, the number that retain fertility for 2 days is sufficiently high in dogs that fertility and fecundity are not affected by matings on the day of the LH surge, 2 days before ovulation. Thus, in such pregnancies, the sperm survived 2 days before potentially penetrating the oocyte, and the chromatin had to survive another 2 to 3 days to function as a male pronucleus which fuses with the female pronucleus to form the 1-cell zygote. Fertility declines with matings earlier than the day of the LH surge (i.e. mating 3 or more days before ovulation). However, litters have occasionally been obtained from forced matings, matings by aggressive males, and artificial inseminations of fresh semen as early as 3 - 5 days before the LH surge. Sperm deposition in such cases was 5 to 7 days before ovulation and at least 7 to 9 days before oocyte maturation.

Another part of the explanation for the large variation encountered in apparent gestation lengths lies with the timing of egg maturation in this species. In dogs (and foxes), unlike most other species, the eggs are still immature when they are ovulated (i.e., they are still primary oocytes) and they do not complete meiosis and become secondary (mature) oocytes until probably 2.5 to 3 days after ovulation. An egg must be a mature, secondary oocyte containing a "female" pronucleus before the "male" pronucleus of a sperm can fuse with it to complete the process of fertilization by forming the nucleus of the new one-cell embryo. In early-mated bitches, a sperm probably penetrates each egg shortly after it is ovulated, but the male pronucleus once formed has to wait for the egg to mature. In late-bred bitches, the female pronucleus of the matured egg is ready to fuse with the pronucleus of a sperm that subsequently penetrates as soon as the male-pronucleus is formed. The interval of nearly 3 days required for oocyte maturation after ovulation has been estimated in at least two ways. One is based on estimating how long after ovulation that matings from different males can still result in pups with different sires. Another is estimation of the time after ovulation at which mating with short-lived frozen-thawed sperm results in pregnancies. Because of this phenomenon of "delayed" oocyte maturation, bitches can readily give birth to litters with multiple sires when there are matings by different males before ovulation.

Timing of Fertilization, Fertility and Fecundity

Thus, it appears that fertilization to the point of nuclear fusion can be accomplished no earlier than about 3 days after ovulation (and, thus, 5 days after the LH surge). Following maturation of the egg to secondary-oocyte status, the fertile life span of an unfertilized egg may be only 1 or 2 days in some instances, since fertility declines if matings are delayed until 4 and 5 days after ovulation (i.e., 6 and 7 days after the LH surge). That is, both litter size and pregnancy rate decline when mating occurs more than 2 days after the maturation of the oocyte. Thus, with a narrow 2-day window for optimal fertilization to occur, it is reasonable that gestation length is consistent when measured relative to the day of the LH surge, or to the day of ovulation. However, some bitches may have one or more fertile oocytes survive to as late as 7 or even 8 days after ovulation which corresponds to 9 or 10 days after the LH surge. While fertility is typically low with matings this late, when pregnancy does occur the gestation length is usually the same that as in other bitches, i.e., with parturition occurring at 64 - 66 days after the LH surge (and 62 - 64 days after ovulation). The above scenario is the basis of well documented cases of bitches with exceptionally short apparent gestation lengths, giving birth to litters as "early" as 55 to 56 days after breeding. Why true gestation length is not always obviously longer in these "late-bred" bitches is not clear, but there are two likely reasons. First, there is as yet unpublished evidence that eggs fertilized 2 days after maturation divide slightly faster than eggs penetrated by sperm before maturation. (Tsutsui, 1999, personal communication) Second, it is likely that the timing of implantation is in part related to a sequence of events regulated by the timing of the changes in serum concentrations of estrogen and progesterone. These do not differ with the time of mating or fertilization or early embryo cleavage rate. It is likely then that there is a very narrow window of time in which the uterus is receptive for implantation. Implantation is estimated to occur at Day 22 - 23 after the LH surge [6]. In some instances of a very late mating, there are anecdotal reports that, because of the resulting small litter size, the fetal signal for parturition is weak, and parturition may be delayed for 1 - 2 days, with an apparent increase in gestation length. However, documented evidence for this has not been published.

It is clinically useful to consider that gestation length in bitches is in most cases 64 - 66 days, when measured as the interval from LH surge to parturition. However, it is important to realize that intervals of 63 and 67 days have been seen in some normal, uncomplicated pregnancies and should not be considered out of the ordinary. Furthermore, there can be error of up to 1 days in estimating the day of the LH surge. Nevertheless, estimating the day of whelping as 65 days after the estimated day of the LH surge can be helpful to dog owners and aid in scheduling whelping management services. Timing the major event of pregnancy from the estimated day of the LH surge can also aid in pregnancy testing and pregnancy management services (Tables 1 and 2).

 

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