How this last menstrual period calculator helps
This tool estimates important cycle and pregnancy dates based on the first day of your last period. It can be useful if you are trying to conceive, tracking ovulation, or estimating an expected due date. Results are based on standard calendar math and common clinical assumptions, including a luteal phase of about 14 days unless you specify otherwise.
What the calculator estimates
- Next expected period: based on your average cycle length.
- Estimated ovulation date: cycle length minus luteal phase.
- Fertile window: 5 days before ovulation through 1 day after.
- Estimated due date (EDD): 280 days (40 weeks) from LMP.
- Current gestational age: counted from LMP to today.
How to use it correctly
1) Enter the correct LMP start date
Use the first day of full menstrual bleeding, not spotting. If you are unsure between two dates, use the one you are most confident about and compare results with your healthcare provider later.
2) Use your real average cycle length
If your cycle is usually 30 days, enter 30 (not 28). Even a 1–2 day difference can shift ovulation and fertile window estimates.
3) Keep luteal phase at 14 unless you track it
Many people have a luteal phase around 12–14 days. If you have tracking data (for example from basal body temperature or ovulation tests), enter your typical number for better estimates.
Understanding your results
Calendar calculators are a planning aid, not a diagnosis. Ovulation does not happen at exactly the same time every cycle, and stress, illness, travel, sleep changes, and medications may shift timing.
- If trying to conceive, focus on the fertile window and have intercourse every 1–2 days during that span.
- If your period is late, take a home pregnancy test according to package timing instructions.
- If pregnant, your clinician may adjust due date after an early ultrasound.
Accuracy limits to keep in mind
Irregular cycles
If cycle length varies widely month to month, any date estimate is less reliable. In that case, ovulation strips, cervical mucus tracking, or clinician guidance may be more useful.
Postpartum or after hormonal contraception
After birth, breastfeeding changes, or stopping hormonal birth control, cycles may take time to normalize. Estimates can be off during this transition.
Medical conditions
Conditions such as PCOS, thyroid disorders, and high prolactin levels can alter cycle timing. If you suspect a hormonal issue, discuss your cycle history with a qualified medical professional.
Quick FAQ
Can this calculator confirm pregnancy?
No. It only estimates dates. Use a pregnancy test and medical care for confirmation.
Why is my due date different from another app?
Different tools may use different assumptions about ovulation timing and rounding. Your provider may update your due date after ultrasound measurements.
Is a 28-day cycle required?
Not at all. Enter your actual average cycle for a better estimate.