The goal of this randomized, double-blind, placebo-controlled clinical trial is to evaluate whether melatonin supplementation improves glycemic control in pregnant women diagnosed with gestational diabetes mellitus (GDM).

The main question it aims to answer is:

Does melatonin supplementation help with glycemic control, especially in lowering fasting plasma glucose level?

Researchers will compare melatonin to a placebo (a look-alike substance that contains no melatonin) to see if melatonin works to improve glycemic control.

Participants will:

1. Take melatonin or a placebo every day after randomization until delivery
2. Visit the antenatal clinic once every 1 to 2 weeks for follow-ups

Inclusion Criteria:

• Women aged 18 to 45 years
• Singleton pregnancy
• A diagnosis of GDM from a 75-g OGTT during 24 to 28 gestational weeks, according to the IADPSG criteria, with at least fasting plasma glucose (FPG) ≥ 5.1 mmol/L
• Intending to receive obstetric care and deliver in the study center
• Willing and able to provide written informed consent and follow the study procedure

Exclusion Criteria:

• Use of melatonin 1 month before pregnancy or/and during pregnancy
• Night shift work or exposed to jetlag on a regular basis during pregnancy
• Contraindications to melatonin use, including hypersensitive or allergic to melatonin
• Use of antidepressive or antipsychotic medications which can interfere with melatonin metabolism and/or elimination, such as fluvoxamine, 5- or 8-methoxypsoralen, cimetidine, quinolones, and other CYP1A2 inhibitors; carbamazepine, rifampicin, and other CYP1A2 inducers; and zaleplon, zolpidem, zopiclone, and other non-benzodiazepine hypnotics
• Pre-pregnancy diabetes, including patients diagnosed with diabetes before conception, fasting plasma glucose ≥ 7.0 mmol/L or HbA1c ≥ 6.5% in the first trimester, typical hyperglycemic symptoms or hyperglycemic crisis with random blood glucose ≥ 11.1 mmol/L
• Other major diseases before gestation, e.g. hypertensive disorders, rheumatology or malignant diseases, infected with hepatitis B or hepatitis C, chronic diseases leading to impaired heart, liver, or renal function
• Major fetal anomalies