Serum melatonin during human pregnancy.

Serum melatonin levels during the third trimester of pregnancy averaged 76.5 pmol/l, significantly higher than in earlier trimesters and non-pregnant women.

• Serum melatonin concentrations were measured in 55 pregnant women across three trimesters and 11 non-pregnant women.
• Levels during the first and second trimesters were 29.7 pmol/l and 39.1 pmol/l, respectively.
• A positive correlation was observed between gestational week and serum melatonin at 11.00 h.
• A clear daily pattern of melatonin levels was identified in both early and late pregnancy.
• In late pregnancy, the nocturnal rise in melatonin was higher in amplitude and duration without a phase shift.
• Increased melatonin levels in late pregnancy may result from enhanced production or slower breakdown.

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