Sex-bias in utero alters ovarian reserve but not uterine capacity in female offspring

Environmental stressors to which a foetus is exposed, affect a range of physiological functions in post-natal offspring. We aimed to determine the in-utero effect of steroid hormones on reproductive potential of female offspring using a porcine model. Reproductive tracts of pigs from female-biased (>65% female, n = 15), non-biased (45–54.9% female, n = 15), and male-biased litters (<35% females, n = 9) were collected at slaughter (95–115 kg). Ovaries and uterine horns were processed for H&E or immunohistochemistry. Variability of data within groups was analysed with a Levenes test, whilst data was analysed using linear models in R. In the ovarian reserve, there was a significant birth weight by sex ratio interaction (p = 0.015), with low-birth-weight pigs from male-biased litters having a higher number of primordial follicles and the opposite trend seen in pigs from female-biased litters. Sex bias held no effect on endometrial gland development. A lower BW decreased the proportion of glands found in the endometrium (p = 0.045) and was more variable in both male-biased and female-biased litters (p = 0.026). The variability of primordial follicles from male-biased litters was greater than non- and female-biased litters (p = 0.014). Similarly, endometrial stromal nuclei had a greater range in male- and female-biased litters than non-biased litters (p = 0.028). A crucial finding was the greater variability in both primordial follicles in the ovaries from females derived from male-biased litters and stromal cell count in the endometrium of females from male- and female- biased litters. This could be inflating the variability of reproductive success seen in females from male-biased litters.