In experiment 1 we investigated the accuracy of transrectal ultrasonography (TUS) to assess the number (OR) and diameter of corpora lutea (CL) in 45 and 25 sows, respectively, at 23.4 ± 2.9 d of pregnancy. The diameter was calculated as the average diameter of 10 biggest CL. Sows were subsequently slaughtered and OR was assessed by dissection of CL from both ovaries (n = 45) and average diameter of the 10 biggest CL was also calculated after measurement of CL with the caliper rule (n = 25). There was a weak relationship between OR counted after dissection of the ovaries and OR counted with TUS (β = 0.28 ± 0.01 CL/CL, P = 0.01), but there was a strong relationship between the average CL diameter measured with the caliper rule after dissection and the average CL diameter based on TUS (β = 1.0 ± 0.1 mm/mm, P < 0.0001). This shows that TUS is not a valid method to assess OR in pregnant sows but it is a valid method to assess average CL diameter. In experiment 2, we investigated the relationship between the average CL diameter assessed by TUS (n = 100) at 23.8 ± 2.4 d of pregnancy and average piglet birth weight (BW) and observed an increase of 37.6 ± 17.8 g in piglet BW per mm increase in average CL diameter measured by TUS (P = 0.04). This relationship is probably because larger CL develop from bigger follicles at ovulation, which might have ovulated oocytes of higher quality that developed into embryos with higher growth potential and thus higher birth weight.