Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Seek emergency help if pregnancy is possible and you have severe or worsening lower abdominal pain, fainting, marked dizziness, shoulder-tip pain, difficulty breathing, or heavy vaginal bleeding.
And for anyone, someone who:
is bleeding heavily and it will not stop with firm pressure
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
These symptoms may need urgent treatment.
Arrange same-day medical advice if you have a positive pregnancy test with one-sided pelvic pain or vaginal bleeding, even if the symptoms are not severe.
3
Does any of this apply?
Book an appointment
Book a routine appointment if:
You have missed three periods in a row and are not pregnant
Your periods repeatedly become much less predictable
You have new facial hair, severe acne, nipple discharge, hot flushes, or major weight change
Periods have not started by age 15
The change began after starting a medicine
You are worried about fertility or the effect on your bone health
The article also says
A single late period without concerning symptoms can usually be monitored at home.
Every sign above is quoted from the sofidex article on Missed or late periods. If you are not sure, err upward — it is always reasonable to be checked.