An adjustment of sex ratio of offspring to the conditions present at conception 
is seen in many mammals including horses. This depends on preferential survival 
of male embryos under conditions of high energy intake. In several species, 
growth factors including insulin like growth factor (IGF)-1 have been shown to 
promote embryonic development by decreasing apoptosis and increasing cell 
proliferation. We hypothesized that sex-related differences in IGF-1 expression 
in equine embryos during the phase of maternal recognition of pregnancy might 
exist and thus contribute to preferential survival of embryos from either of 
both sexes under specific environmental conditions. Insulin like growth factor-1 
mRNA expression of in vivo-produced equine embryos on different days of 
pregnancy (Day 8, N = 6; Day 10, N = 8; Day 12, N = 14) was analyzed. Insulin 
like growth factor-1 mRNA expression was evaluated by reverse transcription 
quantitative polymerase chain reaction. The sex of the embryo was determined by 
detection of X-inactivation specific transcript (Xist) RNA and equine sex 
determining region of the Y chromosome DNA. Embryos positive for Xist expression 
were classified as female, and Xist negative and equine sex determining region 
of the Y chromosome positive embryos were classified as male. From 28 embryos 
tested, 15 (54%) showed positive Xist expression and were thus classified as 
female. Insulin like growth factor-1 mRNA expression was influenced by sex (P = 
0.01) but not by day of pregnancy (relative expression of IGF-1 in relation to 
β-actin, Day 8: male 5.1 ± 2.1, female 11.4; Day 10: male 5.2 ± 1.6, female 17.4 
± 6.7; Day 12: male 2.6 ± 0.3, female 11.6 ± 2.4). Results demonstrate an 
increased expression of IGF-1 in female equine embryos. Sex-related influences 
on expression of the IGF system are probably related to a gradual X chromosome 
inactivation.