Frequently Asked Questions
Common questions about due date calculations, pregnancy dating, and using our tools.
How accurate is the due date calculator?
Our due date calculator provides estimates based on established medical formulas (Naegele's Rule for LMP, ASRM guidelines for IVF). However, only 4-5% of babies are born on their exact due date. The normal delivery window is 37-42 weeks.
Early ultrasound dating (8-13 weeks) is the most accurate method. Always confirm your due date with your healthcare provider.
What's the difference between LMP and conception date methods?
LMP (Last Menstrual Period): Calculates 280 days from the first day of your last period. This is the most commonly used method because most people know their period date.
Conception Date: Calculates 266 days from the estimated conception date. If you know your exact conception date (e.g., from IVF or fertility tracking), this method is more accurate.
The 14-day difference accounts for the typical ovulation timing after LMP.
How is IVF due date calculated differently?
IVF due dates are calculated from the embryo transfer date, not LMP:
- Day 3 embryo transfer: Transfer date + 263 days
- Day 5 blastocyst: Transfer date + 261 days
- Day 6 expanded blastocyst: Transfer date + 260 days
This is more precise than LMP because we know exactly when fertilization occurred and the embryo's age at transfer.
Why does my due date change?
Due dates can change based on early ultrasound measurements (8-13 weeks), which are the most accurate for dating. If the ultrasound shows the baby is smaller or larger than expected for the LMP-based date, your provider may adjust the due date.
This is normal and clinically appropriate. It's better to have an accurate due date than to stick with an incorrect one based on LMP alone.
Is my data stored on your servers?
No. All calculations happen entirely in your web browser. Your health data — including last period dates, conception dates, IVF transfer dates, and cycle information — is never sent to our servers, never stored in a database, and never shared with third parties.
We use local storage only if you choose to save calculations, and this data stays on your device. You can clear it at any time through your browser settings.
Can I use these calculators if I'm not pregnant?
Yes! The cycle tracker and fertility window calculators are designed for people who are trying to conceive. They can help predict ovulation, identify your fertile window, and track cycle patterns.
However, these are estimation tools, not diagnostic devices. If you're having difficulty conceiving, please consult a reproductive endocrinologist or your healthcare provider.
What if my cycle length isn't 28 days?
Our due date calculator allows you to adjust for your actual cycle length. The standard calculation assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator adjusts the due date accordingly.
For example, a 35-day cycle would push the due date later by about 7 days compared to the standard calculation.