What LMP dating assumes about your cycle
A menstrual cycle runs from the first day of bleeding of one period to the first day of bleeding of the next, and the average cycle lasts 28 days.2 The standard due-date convention counts 280 days from the first day of the LMP and assumes a regular 28-day cycle with ovulation on the 14th day.1
Because of that assumption, ACOG notes that the method does not account for irregularities in cycle length or for variability in the timing of ovulation.1
Long, short and irregular cycles
Cycles vary widely between people: a cycle may be as short as 21 days or as long as 35 days.3 It helps to separate two situations:
- Regular but not 28 days. Your cycle is about the same length every month, just longer or shorter than 28 days. The LMP method still has a fixed reference point, but its 14-day assumption is off for you.
- Irregular. Your cycle length changes from one month to the next, for example 21 days one month and 35 the next. Then it is harder to estimate the due date accurately from the LMP at all.3
Tracking the first day of each period, on a calendar or in an app, shows which of these describes you, and gives your care team useful information.
How cycle length moves the due date
The time between ovulation and the start of the next period is the most consistent part of the cycle; in an average 28-day cycle, ovulation happens about 14 days before the next period.2 So the length of the cycle mostly changes the time before ovulation. ACOG gives examples: in a 21-day cycle, ovulation is likely around day 7; in a 35-day cycle, around day 21.4
Example: same LMP, different cycles
Two people both have an LMP of August 1, 2026, so the 280-day count gives both the same estimated due date, May 8, 2027.
One has a 28-day cycle, so ovulation is likely around day 14. The other has a 35-day cycle, so ovulation is likely around day 21,4 about a week later than the count assumes. With the longer cycle, the estimated due date will be later.3
This site gives no correction formula for cycle length: how a pregnancy is dated in that situation is a decision for your care team.
Why irregular cycles make the estimate less reliable
With irregular cycles, the day of the LMP is still known, but the day ovulation followed it is not, and that is exactly the part of the count the convention assumes. Ovulation timing can be unpredictable even in regular cycles: in a study of 221 women, only about 30% had their fertile window entirely within days 10 to 17 of the cycle.5
The same uncertainty affects a conception date worked out from the LMP; see LMP vs. conception date.
What this site's calculators do
The site's calculators use only the first day of your LMP. They do not ask for your cycle length and do not adjust for it: the Due Date Calculator always adds 280 days, which assumes a 28-day cycle. If your cycles are long or irregular, treat the result as a rough starting estimate, not a date to plan around.
How your care team can date the pregnancy
Pregnancy, Birth and Baby lists a very long or irregular cycle among the situations in which an early ultrasound may be strongly recommended, and suggests a first antenatal visit before 10 weeks.3 An ultrasound of the embryo or fetus in the first trimester is the most accurate method to establish or confirm gestational age, and ACOG notes that a first-trimester ultrasound can improve the accuracy of the due date even when the first day of the LMP is known.1
Ultrasound dating vs. LMP dating explains how a scan date is compared with an LMP date.
This site cannot interpret your dates
Only a healthcare professional can date your pregnancy. Read the medical disclaimer.
Get a starting estimate
The Due Date Calculator counts 280 days from your LMP. With irregular cycles, treat its answer as a rough first estimate.
Common questions
Can I still use a due date calculator if my periods are irregular?
You can, but the result is less reliable, because the 280-day count assumes a regular 28-day cycle.1 Use it as a rough first estimate and tell your care team your cycles are irregular.
Will a longer cycle make my due date later?
Usually. With a longer cycle ovulation tends to come later, so the estimated due date will be later.3
What should I tell my doctor or midwife?
The first day of your last period if you know it, how long your cycles usually are, and whether they change from month to month. A record of past period dates helps. What if you don't know your LMP? has a fuller list.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date (Committee Opinion No. 700). 2017; reaffirmed 2025.
- American College of Obstetricians and Gynecologists (ACOG). Fertility Awareness-Based Methods of Family Planning. patient FAQ, reviewed 2025.
- Pregnancy, Birth and Baby (Healthdirect Australia, funded by the Australian Government). Working out your due date. reviewed August 2024.
- American College of Obstetricians and Gynecologists (ACOG). Trying to Get Pregnant? Here's When to Have Sex. reviewed 2025.
- BMJ. Wilcox AJ, Dunson D, Baird DD. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. 321(7271):1259–62. 2000.