ImagingHigh evidence 7 min readLast reviewed: 2025-05-30
Ultrasound pregnancy diagnosis in the bitch
When to scan, what to see, and why counting fetuses is unreliable.
Transabdominal B-mode ultrasound is the reference standard for early pregnancy confirmation, viability assessment and gestational aging in the bitch. Fetal counting, however, remains unreliable and should not be used to plan the whelping.
Key evidence
- Gestational vesicles become reliably detectable at 21–25 days post-LH surge.
- Fetal heartbeat is typically visible from day 23–25 post-LH and is the most sensitive viability marker.
- Radiography from day 45 onwards is the most reliable tool for litter size estimation.
- Ultrasound aging by fetal biometry has an accuracy of ±3 days in mid-gestation.
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Scanning technique
A high-frequency linear or micro-convex probe (7.5–10 MHz) is preferred. The bitch is scanned in dorsal or lateral recumbency after abdominal clipping. Systematic evaluation of both uterine horns and the bladder-cervix landmark is essential.
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What to document
Number of gestational sacs seen, fetal heart-rate pattern and trend, fetal biometry for aging, presence of any pathology, and maternal structures. Historical studies have used absolute FHR bands (for example >220 bpm as reassuring and <180 bpm as severe concern), but interpretation should account for gestational stage, trend, technique, and the full maternal-fetal context.
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Limits and referrals
Fetal counting is systematically underestimated because of superposition. Marked or persistent FHR reduction, a downward trend, or other suspected pathology warrants prompt veterinary reassessment in context rather than interpretation from one isolated number.
References
- England GCW, Yeager A, Concannon PW. Ultrasound imaging of the reproductive tract of the bitch.
- Kutzler MA et al. Determination of gestational age in the bitch using ultrasonographic fetal measurements. Theriogenology.