Introduction Small-conductance Ca2+-activated K+ channels (SK channels) have been described in the myometrium from non-pregnant humans, and the expression of the channels has been shown to be down-regulated in pregnancies at term (1,2,3). It has been suggested that expression of SK channels may be important for the relaxation of the myometrium during pregnancy, and decreased expression may be a prerequisite for contractions required at delivery. The aim of our study was to investigate the expression of the SK channel isoforms SK2 and SK3 in human pregnant myometrium in term and in preterm deliveries. Materials and methods Human myometrial tissue was obtained in connection with hysterectomy from non-pregnant women (NP) and from pregnant women during caesarean section at term (late pregnant (LP), 38-42 weeks of gestation) and at preterm deliveries (preterm (PT) 27-32 weeks of gestation). Expression of SK2 and SK3 (mRNA levels) were determined by qPCR as ratio to β-actin. Western Blot was used to investigate the channel expression at the protein levels. Results qPCR showed a significantly lower expression of SK2 gene in non-pregnant myometrium as compared to SK3 (normalized value of SK3 was set to 1.00 ± 0.23 (mean ± S.E.M) and that of SK2 was found to be 0.081 ± 0.002, p =0.0005, using Mann-Whitney U test, n = 9 and 11 for SK2 and SK3 respectively). The level of SK2 mRNA showed no significant change in the pregnant tissue compared to the non-pregnant (p = 0.673 using Mann-Whitney U test, n = 11 and 8 for NP and LP respectively). This was confirmed on protein-level with western blot (n = 4 for NP and LP). The expressions of SK3 gene in late pregnant and pregnant with preterm deliveries were both significantly lower than the expression in non-pregnant (normalized expression of 0.152 ± 0.002 for PT and 0.132 ± 0.026 for LP. Tested with Mann-Whitney U test, p = 0.0127 for NP versus PT, and p = 0.0005 for NP versus LP, n = 11, 5 and 12 for NP, PT and LP respectively). This was confirmed on protein level (n = 4 for NP and LP). Conclusion The present study shows that SK3 is the dominant SK isoform in the human myometrium. The level of SK2 expression is insignificant as compared to the level of SK3 in the non-pregnant tissue. The expression of SK3 is strongly down-regulated during the time of delivery in term, but surprisingly the down-regulation appears to take place sometime before week 27 to 32. This indicates that SK3 is not contributing to the relaxation of the myometrium in the last weeks of pregnancy.