Why irregular cycles cause problems
LMP dating depends on ovulation happening at a predictable point. If your cycle ranges from 26 to 45 days, ovulation might have happened anywhere from day 12 to day 31 – a window of nearly three weeks. A due date calculated from the LMP alone could be significantly wrong, which in turn affects the timing of screening tests and decisions near term.
What to do
- Use your average cycle length in the LMP calculator as a rough first estimate.
- If you tracked ovulation (LH strips, follicle study, temperature), use the conception date instead – it’s more precise.
- Get an early dating scan between 7 and 13 weeks. This is the single most important step.
- Once you have the scan result, use the Ultrasound tab or your doctor’s EDD for all future calculations.
PCOS and pregnancy dating
Polycystic ovary syndrome affects a substantial share of women of reproductive age in India. It often causes long, irregular cycles and delayed ovulation, so the baby frequently measures “behind” LMP dates. If you conceived with ovulation-induction medicines, your fertility clinic will have a precise ovulation or trigger date – ask for it. PCOS also raises the risk of gestational diabetes, so early glucose screening is especially important.
Breastfeeding and after the pill
It’s possible to conceive before your first period returns while breastfeeding, or before cycles settle after stopping hormonal contraception. In these cases there may be no usable LMP at all – an ultrasound is the only reliable way to date the pregnancy.
Frequently asked questions
Can I get pregnant with irregular periods?
Yes. Irregular cycles make it harder to predict ovulation, but many women with irregular periods conceive naturally or with simple treatment.
Is ultrasound dating accurate for irregular periods?
Yes. A first-trimester scan is accurate to within about 5–7 days, regardless of your cycle pattern.
Medical disclaimer: The results and information on this page are estimates for general education only. They are not a diagnosis and do not replace advice from your gynaecologist, obstetrician or midwife. If you have pain, bleeding, reduced baby movements or feel unwell, contact your doctor or call 108 immediately. Read the full disclaimer.