How is my due date calculated?
The standard method is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period. The shortcut is to add seven days, subtract three months, and add one year — for example, an LMP of January 1, 2026 yields an EDD of October 8, 2026. This calculator also adjusts for cycle length so that women with non-28-day cycles get a more realistic estimate.
Why is the due date based on LMP and not conception?
LMP is a known, observable date for most women, while exact conception is rarely known. Because ovulation typically occurs about 14 days after LMP, the convention adds those two weeks at the start. This is why you are described as 'six weeks pregnant' even though the embryo is only about four weeks old. Switching to a conception-based count would create constant confusion between patients and clinicians.
What if my cycle is irregular or not 28 days?
Naegele's rule assumes a 28-day cycle. Cycles longer than 28 push ovulation (and conception) later, so the EDD shifts later by the same number of days. Cycles shorter than 28 pull the EDD earlier. This calculator applies that adjustment. For genuinely irregular cycles, after recent hormonal contraception, with PCOS, or while breastfeeding, LMP-based dating becomes unreliable and an early ultrasound is the better dating tool.
Why did my doctor change my due date?
Per ACOG Committee Opinion 700, if a first-trimester ultrasound (before 14 weeks) gives a date that differs from the LMP-based estimate by more than the ultrasound's confidence interval — typically 5-7 days — the ultrasound date should be adopted. Ultrasound dating is significantly more accurate than LMP recall, particularly when the LMP date is uncertain or cycles are irregular. The new date is then used for all subsequent prenatal care.
What does 'full term' mean now?
In 2013 ACOG and the Society for Maternal-Fetal Medicine refined the definitions: early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and post-term is 42 weeks 0 days and beyond. The change was made because outcomes at 37-38 weeks are measurably worse than at 39-40 weeks, even though both used to be lumped together as 'term'.
What if I deliver before or after my due date?
Most babies arrive in a roughly four-week window centered on the EDD. About 80% are born between 38 and 42 weeks. Only 4-5% are born on the EDD itself. Births before 37 weeks are considered preterm and may require NICU support depending on how early. Pregnancies that reach 41 weeks are usually monitored closely (non-stress tests, biophysical profiles), and most providers recommend induction by 41-42 weeks because post-term placental function declines.
How accurate is Naegele's rule?
When the LMP is known and the cycle is regular and close to 28 days, Naegele's rule is accurate to within about plus-or-minus 2 weeks. It systematically tends to overestimate gestation length slightly compared with ultrasound dating in modern populations. Accuracy degrades quickly for irregular cycles, uncertain LMP recall, or cycles outside the 26-30 day range. First-trimester ultrasound is now the gold standard whenever LMP dating is uncertain.
Should I trust my dating ultrasound over the LMP date?
Generally yes, when the ultrasound is done in the first trimester. Crown-rump length measurements before 14 weeks date a pregnancy to within roughly 5-7 days. Second-trimester ultrasounds are less precise (within 1-2 weeks), and third-trimester ultrasounds should never be used to redate a pregnancy. If your first-trimester ultrasound and LMP disagree by more than the ultrasound's confidence interval, ACOG recommends using the ultrasound date.
Does this calculator work for IVF or known conception dates?
This tool uses the LMP method, so it is not the most precise option for IVF pregnancies. For IVF, providers add 266 days to the conception date, then adjust for the embryo's age at transfer (a day-5 blastocyst already counts as 5 days of development). The result is much more precise than LMP-based dating because the conception date is known to the hour. If you conceived through IVF, ask your clinic for the official EDD.
Can a pregnancy go past 42 weeks?
It can, but it is uncommon and increasingly discouraged. After 42 weeks, the risks of stillbirth, meconium aspiration, macrosomia, and placental insufficiency rise. Most providers offer induction between 41 and 42 weeks if labor has not started spontaneously. Whether to induce, when, and how is an individualized decision based on cervical readiness, baby's well-being, and your own preferences — discuss the options with your OB-GYN.
