Why Is Period Flow Heaviest on the First Day? Hormonal Changes, Clots, and Contractions

Heavier flow on day 1 is physiologically normal for many women. Contributing factors:

1. Hormonal withdrawal: Progesterone/estrogen drop triggers rapid shedding of the thickened endometrial lining accumulated during the luteal phase.
2. Blood clots: Pooling in the uterine cavity overnight forms clots expelled in the morning.
3. Uterine contractions: Prostaglandin-induced contractions are strongest initially, expelling more tissue.
4. Vasodilation: Spiral arteries dilate, increasing blood flow to the endometrium.
5. Cervical position: Slightly more open at onset, facilitating passage.

When to see a doctor: Soaking a pad/tampon in <1 hour, passing clots >1 inch, periods >7 days, or symptoms of anemia (fatigue, dizziness). These suggest menorrhagia requiring evaluation.