It is known that changes of sexual steroid hormones during cycle and early pregnancy affect maternal metabolism. But when do embryo-factors start to take influence? The aim was to investigate metabolic changes in cattle already on Day 18. Blood samples of Holstein Friesian heifers (n=30) were taken on Day 0 (Day of ovulation), 7, 14, 16 and 18 in repeated consecutive pregnant and non-pregnant periods. Concentrations of progesterone (P4[ng/ml]), estrogens (E), insulin, insulin-like-growth-factors (IGF-I+IGF-II), triiodothyronine (T3) and thyroxine (T4) were determined. Heifers were defined as pregnant (D18=P4>2.0 and either recovery of conceptus on D18(n=12) or embryonic heart beat on D42 (n=21) [PP4; n=33]) and non-pregnant (D18=P4<2.0 and no conceptus [NP4;n=59]). In several non-pregnant heifers P4 remained high, whichled to the third group (D18=P4>2.0 and no conceptus [NP4;n=9]). Insulin, IGF-I, IGF-II, T3 and T4 blood levels were similar between the three groups at D18. In all groups IGF-I decreased from D0 to D16 (p<0.001) whereas IGF-II increased (p<0.01). Unexpectedly, IGF-I levels increased from D16 to D18 in the diverse groups PP4 and NP4 (p<0.05) but remained stable in NP4. T4 and T3 levels decreased from D0 to D14 and from D0 to D18 respectively in PP4 and NP4(p<0.05). In conclusion the present data confirm literature concerning the fact, that IGF-I increases during estrus. However, despite inverse sexual steroid hormone influence, pregnant heifers showed the same increase in IGF-I towards D18 as non-pregnant heifers with low P4. These observations indicate that in early pregnant heifers IGF-I blood levels might be independent from steroid balance, suggesting that other signals including embryo factors may regulate the production of this growth factor.