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Wednesday, April 13, 2016

Pregnancy; 25 to 27 days

Keywords: canine, pregnancy, 25, 27, diagnosis, surgery, polyovular, fetus,embryo,placenta

The image shows a canine pregnancy estimated to be 25 to 27 days of gestation. This estimation was based on the crown rump length of the embryos and the size of the embryo-placental bulges. There is little information on actual crown-rump length in postmortem specimens of known gestation, therefore estimates were extrapolated from ultrasonographic data. The size of the bitch was unknown and could have belonged to a small or giant breed, another factor that could contribute to inaccuracy of this estimation.

Image size: 1962 x 1198px

The author repeatedly refers to embryos in this entry, not fetuses. That is because  organogenesis is not yet complete and the embryo is not yet recognizable as a the species of interest; two factors occasionally used to define the transition between an embryo and a fetus. In fact, there is no gestational age when organogenesis is indeed complete (CNS development continues even after birth) and recognition of the species of the embryo-fetus lies in the eye of the beholder. The reader is left with that dilemma. Certainly, refereed data do not suggest firm opinions on this matter.

Several items are of note in this specimen. 

1. The ovaries were carefully dissected and only five corpora lutea were present; yet there were six embryos. This is to be expected in bitches, where a single Graafian follicle can contain several oocytes i.e. Canis familiaris is a polyovular species.  

2. A reminder that bitches (and queens) do not have major (middle) uterine arteries such as those present in ruminants and horses. Therefore in young, non pregnant bitches that do not have excessive abdominal fat, little potential exists for severe hemorrhage when the mesometrium is transected during ovariohysterectomy.

3. Ultrasonography has largely rendered transabdominal palpation for pregnancy redundant. However, palpation of  embryonic-placental bulges are potentially important when ultrasonography is not available. In this regard, the author developed a mantra for students on how to recall when palpation was valuable and for what reason. It also included the advent of mammary development and was known as the "25, 35, 45, 55, 65" guide. At its best it is only approximate, but still potentially valuable. Timing was based on the occurrence of the LH surge and it read as follows:

25 to 35 days: embryonic-fetal bulges are palpable except in obese bitches or those that are extremely muscular. 
Note: After 35 days, the embryonic-fetal bulges lose tone and become more difficult to palpate.
45 days: mammary development is first noted in maiden bitches (Colostrum can only be expressed during the last 2 to 3 days of gestation).
55 days: the fetuses themselves become palpable.
65 days: parturition.

4. Carnivora have intimate (endotheliochorial) placentation, arranged in a band around the embryo/fetus. This is called zonary placentation. The margins of zones of placentation in bitches develop hematomas where heme breakdown products facilitate iron transport into the conceptus. One of these products is biliverdin,  the bright green pigment known as uteroverdin in this context. That pigment is not yet visible in a 25 to 27 day pregnancy but becomes a remarkable feature of late gestation, signaling the onset of whelping and in some cases, retention of the placenta after whelping is complete.

Selected references:

Yeager, A.E. and Concannon, P.W. 1990. Association between the preovulatory luteinizing hormone surge and the early ultrasonographic detection of pregnancy and fetal heartbeats in Beagle dogs
Theriogenology 34:655-665

Michelle A. Kutzler, M.A. 2003. Accuracy of canine parturition date prediction using fetal measurements obtained by ultrasonography. Theriogenology. 60:1309-1317

Lopate, C. 2008 Estimation of gestational age and assessment of canine fetal maturation using radiology and ultrasonography: A review. Theriogenology. 70:397-402