Phone +45 3545 0980 / Fax +45 3545 5064 Email anna@rh.dk Word count: 1798 -Abstract: 228 -Figures: 2 A-M.A., A.M.L., A.S.C., C.J., F.S.J., K.B. and N.E.S. have nothing to declare. Abstract Background: According to animal studies, the testicular Leydig cell hormone insulin-like factor 3 (Insl3) exerts a fundamental role in abdominal testis translocation, which occurs in the beginning of second trimester in humans. Despite this, human prenatal INSL3 production has been poorly investigated.

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... : Amniotic fluid from 91 pregnant women undergoing amniocentesis was analysed for INSL3 and testosterone levels. Data were related to gestational age (15-25 weeks) at the time of amniocentesis and to sex (45 males and 48 females). Results: INSL3 was present in amniotic fluid from all but one of the investigated male fetuses (range: <0.02-0.36 ng/ml, mean± ± ± ±sd: 0.12±0.07 ng/ml), whereas the hormone was undetectable in the female fetuses. Testosterone was significantly higher in male (range: 0.54-1.71 nmol/l, mean± ± ± ±sd: 1.04±0.30 nmol/l) as compared to in female amniotic fluid (range: 0.19-0.50 nmol/l, mean± ± ± ±sd: 0.34±0.06 nmol/l) (p<0.001). In males, there was no correlation between INSL3 and testosterone. A statistically borderline negative association was found between INSL3 and gestational age (p=0.07), whereas the corresponding association was not significant for testosterone (p=0.12). In contrast, testosterone in females correlated positively with gestational age (p=0.02). Conclusion: INSL3 is clearly present in human male amniotic fluid in second trimester, where abdominal testis translocation takes place. In contrast, the hormone is undetectable in female amniotic fluid. Prenatal presence of INSL3 supports the hypothesis that this hormone is essential for testicular descent in humans.