Gestational diabetes mellitus (GDM) has a significantly increased risk of spontaneous preterm delivery (SPD) (Boriboonhirunsarn & Tanpong, 2023). Certain medications used in patients without diabetes to control premature contractions should be used with extreme caution in pregnant women with diabetes because they can significantly affect blood sugar control by causing increases in blood glucose concentrations (e.g., terbutaline/brethine) (Peterson et al., 1993). Therefore, there is a need to find complementary therapies. Our main aim was to examine the impact of Thunbergia laurifolia L. (TL), a well-established anti-diabetic plant (Kosai et al., 2015), on GDM rat myometrium contractility (in vitro) and ultrastructure (in vivo). TL leaves were ethanolic extracted, and the effect was tested. The animal care followed the guidelines of the committee of Care and Use of Laboratory Animal Resources, National Research Council of Thailand. The experiment procedures were approved by the Institutional Animal Care and Use Committee, Suranaree University of Technology, Nakhon Ratchasima, Thailand (Approval no. 13/2560). A single dose of streptozotocin (STZ) 60 mg/kg BW was given to induce GDM in pregnant rats on day 5 of gestation. Blood samples were obtained from a tail vein puncture, and glucose levels were monitored two days after STZ to confirm diabetic induction by a glucometer. Diabetes was defined as hyperglycemia exceeding 200-300 mg/dL. For in vitro study, GDM rats were humanely killed at term, and myometrial strips were isolated for isometric force measurements. For in vivo study, TL was orally and daily administrated at high (500 mg/kg BW) and low doses (50 mg/kg BW) from day 7 of gestation until term, and its effects on blood glucose and myometrial ultrastructure were investigated. The results showed that TL extract significantly inhibited spontaneous uterine contractility in a concentration-dependent manner with IC₅₀ of 1.19 mg/ml (n = 5). The spontaneous force was significantly reduced to 76±8% when compared with 100% control (n = 5). The significant reduction was still active, continuing in combination with KCl depolarization and oxytocin-mediated contractility in both groups. Thus, the force was reduced to 79±7% and 74±7% in the presence of KCL and oxytocin when compared with 100% control (n = 5). TL significantly decreased blood glucose levels in GDM (n = 5). Both high and low doses of TL significantly decreased blood glucose levels to 418 ± 29 and 431 ± 11 mg/dL when compared with GDM non-treated control 588 ± 13 mg/dL. Histological study revealed that the muscular layer significantly increased in thickness in both high and low doses of TL compared with GDM control (67.35 ± 2.89%, 62.30 ± 2.26%, and 52.66 ± 2.36%, respectively). Taken together, TL produced an inhibitory effect on GDM myometrium, irrespective of the type of contractility. Along with the effect of reducing blood glucose levels, TL also restores deleterious GDM myometrium by increasing its thickness. Therefore, TL is worth further investigation in human myometrium and has developed as a tocolytic agent to prevent SPD in GDM.