The first two years are often irregular
Cycles can be anywhere from 21 to 45 days in the first couple of years while hormones settle. Skipping a month occasionally is common and usually fine.
By 3 years post-menarche, cycles should be more regular — 21 to 35 days apart, with bleeding lasting up to 7 days.
Usually normal
• Cramps on day 1–2 that ease with a hot water bottle and ibuprofen.
• A few clots smaller than a 10p coin.
• Mood dips or tearfulness in the week before her period.
• Tiredness or lower appetite on heavy days.
Speak to a GP — soon
• Bleeding through a super pad or tampon in under 2 hours, repeatedly.
• Periods lasting longer than 7 days.
• Missing school every month because of pain.
• Pain that ibuprofen and a hot water bottle don't touch.
• No period by age 15, or no signs of puberty by age 13.
• Bleeding between periods, or after sex once she's sexually active.
Same day — NHS 111 or A&E
• Sudden very heavy bleeding with dizziness, fainting, or a racing heart.
• Severe one-sided pelvic pain.
• A high temperature with tampon use.