Ultrasonographic appearance of the ovary and uterus of the bitch during oestrus, ovulation and early pregnancy.

The ovaries and uteri of ten bitches were imaged daily from the onset of pro-oestrus until 30 days after the plasma surge of luteinizing hormone. During pro-oestrus small follicles increased in size but did not protrude above the surface of the ovary. Ovulation was characterized by a decrease in the number of fluid-filled follicles and their replacement with similarly sized hypoechoic structures. These structures declined in number after ovulation and were replaced by fluid-filled corpora lutea. Gradual thickening of the wall of the corpus luteum occurred with subsequent obliteration of the central anechoic cavity. During pro-oestrus and oestrus there was an increase in the transverse diameter of the uterus, which developed a characteristic appearance with central radiating areas of hyperechogenicity and zones of hypoechogenicity. These uterine changes were not considered to be specific enough to be used as an indication of ovulation. Uterine diameter and echogenicity did not differ between pregnant and non-pregnant bitches until gestational sacs were identified, a mean of 18.6 +/- 0.9 (SD) days after the plasma surge of luteinizing hormone. Embryonic tissue and heart beats were first detected between 23 and 27 days after the pre-ovulatory luteinizing hormone surge.