Journal of Agricultural Science, Cambridge (1999), 133, 227–234. Printed in the United Kingdom  1999 Cambridge University Press 227 Correlations between lamb birth weight and the concentrations of hormones and metabolites in the maternal serum during pregnancy W. MANALU* AND M. Y. SUMARYADI† Department of Physiology and Pharmacology, Faculty of Veterinary Medicine, Bogor Agricultural University, Jalan Taman Kencana I No. 3 Bogor 16151, Indonesia (Revised MS received 31 March 1999) SUMMARY Maternal serum hormones (progesterone, oestradiol, triiodothyronine and cortisol) and blood metabolites (β-hydroxy butyric acid [BHBA] and blood urea nitrogen [BUN]) concentrations at weeks 0, 4, 8, 12, 16 and 20 of pregnancy were measured in 39 pregnant Javanese thin-tail ewes (20 and 19 carrying single and multiple [2–3] foetuses, respectively), and six non-pregnant ewes as controls, to evaluate their correlations with lamb birth weight at parturition. All hormones and metabolites changed with the advance of pregnancy. Regression analyses showed that concentrations of triiodothyronine, cortisol, BHBA, and BUN in the maternal circulation during pregnancy had lower correlations with lamb birth weight at parturition than those of progesterone and oestradiol. Progesterone and oestradiol concentrations in the maternal circulation at week 8 of pregnancy had the greatest correlations with lamb birth weight at parturition. The higher the concentrations of progesterone and oestradiol in the maternal circulation at week 8 of pregnancy, the higher the lamb birth weight at parturition. The results suggested that raising maternal serum progesterone and oestradiol concentrations during early pregnancy in sheep, either by exogenous administration or superovulation, might improve prenatal growth and lamb birth weight. INTRODUCTION Birth weight, which is the resultant of prenatal growth, is a significant factor in the survival of lambs during the preweaning period. In sheep, the larger the litter size, the lower the lamb birth weight, which often results in a higher preweaning mortality. Improvement in prenatal growth could improve birth weight. Prenatal growth is influenced by nutrient supply to the developing embryo and foetus that is facilitated by a suitable environment for the uterus and placenta. Maternal hormonal and nutritional status dictates the suitability of the environment in the uterus and placenta (Harding & Johnston 1995 ; Robinson et al. 1995) Progesterone, along with oestradiol, is known for its effects on uterine and placental growth (Bell 1984 ; Wheeler et al. 1987 ; Ashworth & Bazer 1989 ; Mulholland et al. 1994) during early pregnancy and * To whom all correspondence should be addressed. † Present address : Laboratory of Physiology and Re- production, Faculty of Animal Sciences, Jenderal Soedirman University, P.O. BOX. 110, Purwokerto, Indonesia. for maternal tissue metabolism during late pregnancy (Shirling et al. 1981 ; Sutter-Dub et al. 1981). Secretion of progesterone changes dramatically during preg- nancy and increases with increased litter size (Manalu et al. 1996 ; Manalu & Sumaryadi 1998 a, b). Cortisol regulates foetal growth through its effects on tissue differentiation and maturation, whereas thyroid hormones affect tissue accretion and differentiation (Fowden 1995) and their concentrations increase with the advance of pregnancy (Manalu & Sumaryadi 1996). Maternal lipid and protein metabolism, as indicated by changes in maternal serum concen- trations of beta-hydroxy butyric acids (BHBA) and blood urea nitrogen (BUN), change with the advance of pregnancy (Manalu & Sumaryadi 1998 c). The objectives of the present study were to correlate concentrations of progesterone, oestradiol, triiodo- thyronine, cortisol, BHBA and BUN in the maternal circulations at certain ages of gestation during pregnancy with lamb birth weight at parturition, and to determine when, during the course of pregnancy, variations in those parameters had the greatest correlation with lamb birth weight.