Canine Fetal Development
Canine Fetal development In the majority of female dogs parturition occurs between 64-66 days, after the ovulatory surge. Using the date of mating as a reference point as observed in cases of just a single mating or insemination, birth can occur as early as 56 days later and as late as 68 days later. Part of the explanation is that dog sperm in some cases, survive in the dogs tract for up to 7 or 9 days and still remain viable in terms f 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 retains fertility for 2 days is sufficiently high in dogs that mattings do not affect fertility and fecundity 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 pronuclear, which fuses with the female pronuclear to form the 1-cell zygote. Fertility declines with mating’s earlier than the day of the LH surge (i. e. mating 3 or more days before ovulation).
The beating fetal heart may be seen as a distinct flickering by day 23-24, but may not be easily detected until day 25. U/S equipment of greater or lesser resolution may detect these and other landmark states or events earlier or later, respectively. Changes in embryo diameter, crown rump length, biparietal head diameter, and other parameters have been well characterized, at least for research dogs. These data have also been successfully applied to estimating gestational age clinically on many breeds based on U/S exam findings, and considering a 0-2 day error depending on the size of the animal and other factors (Kutzler et al, 2003).
Relational, non-quantitative changes may be more precise and less breed or animal dependent: e. g. , the time when the fetal crown-rump length first exceeds the length of the placental girdle in research beagles (Day 39-41) is likely the same in most or all other breeds. Canine placentation is endothelial-chorial; the girdle of fetal trophoblastic tissue that forms the zonary placenta and develops marginal hematomas is distinct in U/S exams after day 28. The chorioallantoic poles remain thin and transparent. The marginal hematomas develop large pools of stagnant blood from which the extra-embryonic circulation absorbs metabolites and iron.
Fetal heart rate (HR), typically >220, averages about 230 bpm and can be routinely monitored by most U/S equipment after Day 28. Most experienced veterinarians can determine pregnancy by simply feeling the dog’s abdomen during a certain a window of time—about 20-30 days after conception. During this time period, there is uterine swelling around the placental sites that feels like firm and discrete lumps. After 30 days, the uterine swelling is more diffuse and it is difficult to distinguish the gravid uterus from the feel of the intestinal tract. Dogs that are very large or obese may be difficult to examine in any stage, however.
A third way to detect pregnancy is by taking x-rays. Fetal skeletons can be visualized at about 45 days of pregnancy. This test cannot be done until late in gestation, but it is nice to know about how many of the little creatures you can expect once the actual birthing occurs. After 57 days the fetus has completed prenatal development and is ready to be born, whelping is imminent from this point on. Canine Pregnancy Timeline Fertilization: Germ cells, i. e. sperm and egg fuse together and the phases of mitosis begin. During mitosis genetic information from the parents is merged. 96 Hours:
Divides into two cells. 120 Hours: Divides into four cells. 144 Hours: Divides into eight cells. 192 Hours: The morula is constituted. The morula is a compact mass of 32 cells. In size the mass has grown little. 15 Days: A blastula is constituted. The blastula is a hollow ovule consisting of 500- 2000 cells. The blastula is not attached and floats freely within the uterus. 23 Days: The individual ovules have adhered to the walls of the uterus. Swelling occurs at adherence sites. Palpitation may be used for the next few days to determine pregnancy but the method is only accurate 70% of the time.
A test for the relaxin hormone can be conducted at this point to determine pregnancy. 28 Days: The embryo is oval shaped now and measures approx. 1. 4 cm to 1. 8 cm. 30 Days: The eyes are formed and covered by eyelids. Sexual organs are apparent. 43 Days: Heartbeat is detectible by ultrasound. 45 Days: From this time on the growth of the fetus is accelerated. The mother’s appetite will likely increase. pregnancy. 57 Days: The fetus has completed prenatal development and is ready to be born. 63 Days: Considered the normal full term gestation period.