Although the time between parturition is short, pregnancy diagnosis and estimation of the gestational age should be considered important in rabbit practice. In case of lack of reports of the date of mating, transabdominal ultrasonographic examinations would be used effectively and practically in clinics and farms.
The rabbit uterus is ultrasonographically detectable only in pregnant animals. Averagely 8 mm-diameter fluid-filled embryonic vesicles are seen as anechoic, round shape formations and uterine wall is getting thicker7. Pregnancy diagnosis was performing apparently at days of 7 postmating by seeing vesicles which diameter 10-12 millimeters5,7,8. In this study, embryonic sacs were detectable on 7th day of pregnancy, according to the results of earlier studies5,9. But the conclusive diagnosis was done based on signs of 9th day postmating again in order to the exaclude possibility of the early embryonic losses. Moreover, it could be possible that the Doppler examinations are performed with a microconvex probe with 8 MHz from this day onward.
There have been several detailed studies on fetal somatic growth and also fetal Doppler findings in rabbit5,10,16. Concerning of fetal biometric parameters results reported in previous studies, the agreement between our study's results and others were seen. Regarding the high correlations between the day of the pregnancy and the somatic growth parameters, CRL, BPD and TD are seen as very reliable parameters for determining gestational age in cases with an unknown breeding date. In the present study, BPD was measurable throughout pregnancy and has the highest correlation value with gestational age. Therefore, it can say that BPD is the most reliable and practical biometric parameter in assessment of fetal growth in rabbit.
Fetal heart rates have been extensively analyzed to describe normal development over the course of pregnancy. It is well recognized that FHR decreases along with pregnancy in sheep and goats17,18, and it remains more stable in dogs11 and cats19. But there has been a limited FHR findings in rabbit fetuses. Polisca et al10 observed no significant change in FHR during pregnancy. But in the present study, different FHR curve was recorded. The similar decreasing trend was only observed between 20th and 27th days. In the last examination on 30th day, FHR increased again the prior to parturition. In our opinion, this differences in FHR curve is not attribute to fetal stress related with any pathology due to minimizing of environmental stress factors and also no pathological Doppler findings suggestive of fetal stress (end diastolic block and /or notch formation e.g.) were detected during the study. Therefore, this increase seen in the last days of pregnancy can be related to physiological fetal stress that triggers the parturition process. Another results of Doppler exams revealed that the low success of the estimation of the gestational age based on FHR. In earlier studies, FHR value correlated in a wide range (r= -0.77 and -0.55) with gestational age during pregnancy in Saanen goats18 and Egyptian native goats20. Moreover, there is no data related to these correlations in rabbit fetuses. Therefore FHR, PI and RI values would be inconsistent in rabbit due to low correlations comparing with other biometric parameters.
Following the evaluation of the Doppler examinations, it is possible to say that the findings of 20th day of pregnancy can be accepted as threshold in this study. The umbilical arterial blood flow that has typically low-resistance was characterized only by systolic waveform until the 20th day. On the 20th day of pregnancy, RI decreased significantly (P<0.01) after then, it was stable until parturition. Although PI value has no change significantly, its curve tended to decrease. According to the previous studies in different species12,21-23, diastolic waveform takes places following the decreasing of Doppler indices. The last third of tfe pregnancy is very important and characterized by maximum fetal growth and rate, therefore, due to the need for a higher amount of nutrient24. In our opinion, the resistance in umbilical vessels started to decrease significantly at this stage and fetal blood flow within the umbilical cord can be seen in diastolic phase continuously. In rabbit IUGR models, fetal growth retardation findings could be seen examinations at day 2115. Consequently, the biometric and Doppler examinations findings should be evaluated together in the antenatal assessment of fetal growth in third trimester.
In conclusion, ultrasound monitoring with B-mode and Doppler may be a useful tool for assessment of fetal growth in the second and third trimester in the rabbit. BPD should be the first choice for estimation of gestational age and can be used as an important parameter for fetal growth retardation. Although umbilical artery blood flow shows some similarity with other species especially in the last trimester, futher studies should be undertaken to evaluate the feto-maternal hemodynamic changes. The findings herein could be interest to researchers studying the rabbit reproduction and also fetal monitoring and may be used in the future.
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