Period 18 Days Late: Pregnant or Something Else?

Testing & late periods. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

If your period is 18 days late, pregnancy is one possible reason, but far from the only one. Here's what to do next.

Should you test?

At 18 days late, a home test will almost certainly be accurate. If it's negative and your period still hasn't come, see a doctor to look for other causes. Missing three periods in a row, when you're not pregnant, should always be checked.

Common reasons for a late period

• pregnancy
• stress or a big change in routine
• later ovulation than usual, which pushes the whole cycle back
• illness
• significant weight loss or gain, or intense exercise
• starting or stopping hormonal contraception
• breastfeeding
• polycystic ovary syndrome (PCOS) or thyroid conditions
• perimenopause

Working out your dates

If you track your cycle, the pregnancy test date calculator shows when your period was due and the best day to test. If you get a positive result, the due date calculator gives your estimated due date.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.