When Will You Meet Your Baby? The Science of Due Dates
Pregnancy is a journey of 40 weeks, but pinpointing the exact finish line can be confusing. Doctors typically use the "First Day of Last Period" (LMP) to estimate your due date, assuming a perfect 28-day cycle. However, real bodies are rarely that predictable. Our Advanced Pregnancy Calculator offers three distinct calculation methods—including specialized algorithms for IVF (In Vitro Fertilization) transfers—to give you the most precise Estimate of Delivery Date (EDD) possible.
Which Method Should You Choose?
Accuracy depends on the data you have. Choose the method that fits your situation:
- Last Period (LMP): The standard medical method (Naegele's Rule). It adds 280 days (40 weeks) to the first day of your last cycle. Best if you have regular periods and conceived naturally.
- Conception Date: If you were tracking ovulation and know exactly when the baby was made, this is actually more accurate than LMP. It adds 266 days (38 weeks) to the date of intercourse/ovulation.
- IVF Transfer: Critical for fertility patients. Since the embryo was already 3 or 5 days old when transferred, standard calculators will be wrong. This mode subtracts the embryo age (3-day or 5-day blastocyst) to calculate the true gestational age.
Gestational Age vs. Fetal Age
"Pregnancy Math" is weird. If you are 4 weeks pregnant, the baby has only existed for 2 weeks. Why?
- Gestational Age: Doctors start the clock from the first day of your period (before you were even pregnant!). This is the standard "Week 1, Week 2..." count used in apps and medical charts.
- Fetal Age: The actual age of the growing baby, starting from the moment of fertilization. It is always exactly 2 weeks behind the Gestational Age.
Key Milestones Timeline
Knowing your due date helps you prepare for the big moments:
- First Trimester (Weeks 1-12): The baby's heart starts beating (Week 6) and all major organs form.
- Second Trimester (Weeks 13-26): You may find out the gender (Week 20 scan) and feel the baby move.
- Third Trimester (Weeks 27-40): The baby gains weight rapidly and prepares for birth.
Frequently Asked Questions
Is the due date a guarantee?
No. Only about 4% of babies are born on their exact due date. Most are born within a two-week window before or after that date (Weeks 38 to 42). Your due date is an estimate, not a deadline.
What if my cycle is not 28 days?
If you have a short cycle (e.g., 25 days) or a long cycle (e.g., 35 days), the standard LMP calculation will be off by several days. Our calculator allows you to adjust the "Average Cycle Length" to correct this error.
Can an ultrasound change my due date?
Yes. An early dating ultrasound (usually around 8-10 weeks) is the most accurate way to date a pregnancy. If the ultrasound date differs from your LMP date by more than a week, doctors will usually officially change your due date to match the ultrasound measurements.