Background
Melatonin is a potent oxygen scavenger and is capable of altering blood flow in various vascular beds.

Aims
We aimed to determine the effect of melatonin on ovarian vascular indices during ovarian stimulation for in vitro fertilisation (IVF).

Materials and Methods
This is a pilot double‐blind placebo‐controlled randomised trial. Sixty‐nine women (mean age 35.8 ± 4.3 years) undergoing their first cycle of IVF were randomised to receive either placebo, 2, 4 or 8 mg of melatonin, twice a day. Each participant underwent a transvaginal ultrasound at days 6–10 assessing follicular number and size. The vascularisation index (VI), flow index (FI) and vascularisation‐flow index (VFI) were measured. These indices were then correlated with embryological outcomes. Informed consent was obtained from participants. This trial was registered with the Australia New Zealand Clinical Trials Registry (ACTRN12613001317785).

Results
The number of follicles did not differ between groups (P = 0.4). There were no differences in the VI (P = 0.4), FI (P = 0.1) or VFI (P = 0.3) in the right ovary or the FI (P = 0.3) or VFI (P = 0.3) in the left ovary between groups. When comparing placebo to any dose of melatonin, there were no differences in any measured parameter. While there was correlation between the number of follicles on ultrasound and all measured embryological outcomes, there was no correlation between ovarian vascular indices and these important clinical outcomes.

Conclusions
Melatonin does not appear to change ovarian vascular indices during ovarian stimulation. In addition, such vascular indices cannot predict the number or quality of oocytes or embryos obtained in an IVF cycle. These findings require confirmation in future larger studies.