Pregnancy dating guide
How Is a Due Date Calculated?
Most due date estimates start with the first day of your last menstrual period, often called LMP. A typical pregnancy is dated as about 40 weeks, or 280 days, from that date. This does not mean conception happened on the first day of bleeding; it is a standard way clinicians count pregnancy weeks because many people know their period date more reliably than their exact ovulation or conception date.
Naegele's Rule
Ovucal uses Naegele's Rule for LMP-based estimates: add one year, subtract three months, and add seven days to the first day of the last menstrual period. In simple calculator form, that is the same as adding about 280 days to the LMP date.
Conception Date Method
If you know the likely conception date, the calculator can estimate the due date by adding about 266 days. This method can be useful when ovulation was tracked with tests, fertility treatment, or a clearly known conception window. If the date is uncertain, the LMP method is often easier to use.
Why the Result Is Only an Estimate
A due date is not a scheduled delivery date. Many healthy pregnancies deliver before or after the estimated date. Cycle length, ovulation timing, early pregnancy measurements, and individual health factors can all affect dating. Your clinician may use ultrasound, menstrual history, and medical context to choose the official estimated due date.
When to Contact a Healthcare Professional
- You have bleeding, severe pain, fainting, fever, or other urgent symptoms.
- Your cycles are very irregular or you are unsure of your last period date.
- You conceived through fertility treatment or need medication guidance.
- You need confirmation of pregnancy dating, gestational age, or prenatal screening timing.
FAQ
Can my due date change?
Yes. A clinician may adjust the estimated due date after reviewing early ultrasound measurements or more complete cycle information.
Is LMP dating reliable for irregular cycles?
It can be less reliable when ovulation does not occur near the middle of the cycle. If your cycles are irregular, medical dating may provide a better estimate.
Does this calculator confirm pregnancy?
No. It only estimates dates. Use a pregnancy test and contact a healthcare professional for confirmation and prenatal care.