Ultrasound monitoring of corpus luteum morphological evolution and serum progesterone concentration in pregnant and non-pregnant dogs: A prospective, observational study

The corpus luteum is the only structure producing progesterone during pregnancy in dogs. The aim of this study was to characterise morphological changes of corpora lutea in the bitch and assess their relationship with body weight, serum progesterone concentration, and multiple resorptions. We monitored 26 bitches weekly from ovulation confirmation to 35 days post-ovulation, measuring the corpora lutea diameter via ultrasound examination in combination with progesterone assays. The pregnancy rate was 80.7% (21/26), and all pregnancies were carried to term. Dogs were classified into small (5-15 kg), medium (16-39 kg), and large breeds (40-65 kg). Dog weight significantly influenced mean luteal diameter (P < 0.001), which ranged from a mean +/- SD of 3.4 +/- 0.5 mm for small dogs to 6.0 +/- 0.7 mm for large dogs on the day of ovulation confirmation. From ovulation confirmation to peak, corpora lutea grew significantly (2.1 +/- 1.2 mm; P < 0.001) and returned to their initial size by day 35. Surprisingly, one-third of maximum corpora lutea exceeded 1 cm before undergoing subsequent physiological diametric reduction. This growth in luteal diameter was positively correlated with serum progesterone concentration (P < 0.05). This study provides novel findings on canine corpus luteum characteristics, not previously described in literature, which could aid ovulation detection and differentiation between physiological and potentially pathological structures.