However you know your pregnancy dates, whether from your last period, a due date given by your provider, a known conception or IVF transfer date, or an ultrasound measurement, this gestational age calculator works from whichever one you have. Pick your dating method, and it returns your current gestational age, due date, countdown, and a visual sense of how far along you are.
How to use this gestational age calculator
- Choose a dating method tab: Last period, Due date, Conception, or Ultrasound.
- Last period: enter the First day of last menstrual period and your Average cycle length (dating adjusts automatically for cycles shorter or longer than 28 days).
- Due date: enter your Estimated due date directly.
- Conception: enter the Conception/IVF transfer date and select the Type (natural conception/ovulation date, IVF day-3 transfer, or IVF day-5 transfer).
- Ultrasound: enter the Ultrasound date and the gestational age shown at that scan (weeks + days).
- Set “Calculate as of” to today (default) or another date.
- Click “Calculate gestational age.”
- Review your Gestational Age, Estimated Due Date, Countdown, Pregnancy Complete percentage, Total Days Along, Estimated Conception date, and Next Milestone.
- Check the Sonogram View for a visual representation, and export your report as PDF, or share/copy it.
What this gestational age calculator does
Rather than assuming you only know your last period date, this calculator accepts four different starting points and converts any one of them into the same complete set of results. This matters because not everyone dates a pregnancy the same way, someone with irregular cycles might only trust an ultrasound measurement, while someone who conceived via IVF has an exact, known transfer date that’s more precise than an LMP-based estimate would be.
The four dating methods explained
| Method |
How it works |
Best used when |
| Last period (LMP) |
Counts forward from the first day of your last period, adjusted for your actual cycle length rather than assuming a fixed 28 days |
You have a reliable, remembered LMP date and reasonably regular cycles |
| Due date |
Works backward from a due date already given to you (by a provider or an earlier calculation) |
You already have a confirmed due date and just want the current gestational age from it |
| Conception / IVF transfer |
Counts forward from a known conception or embryo transfer date, adjusted for embryo age at transfer (day-3 vs day-5) |
You know your exact conception date, or conceived via IVF with a known transfer date |
| Ultrasound |
Uses the gestational age measured at a specific scan date to calculate forward or backward from that known reference point |
Your LMP is uncertain or your cycles are irregular, making ultrasound dating the most reliable option |
Why cycle length matters for LMP dating
The Last period method doesn’t assume every cycle is exactly 28 days. Entering your actual Average cycle length adjusts the underlying ovulation and conception timing accordingly, since a longer or shorter cycle shifts when ovulation likely occurred relative to your last period, which in turn shifts the calculated gestational age and due date.
IVF dating: why embryo age at transfer matters
For IVF pregnancies, the Type selector distinguishes between a natural conception/ovulation date, a day-3 embryo transfer, and a day-5 embryo transfer. A day-5 (blastocyst) embryo is already 5 days further along in development at the moment of transfer than a day-3 embryo would be, so the calculator adjusts its dating math accordingly rather than treating every transfer date the same way regardless of embryo age.
Reading your results
Beyond the headline Gestational Age and Estimated Due Date, the results include a Countdown to your due date, the Pregnancy Complete percentage (a quick visual sense of overall progress), Total Days Along, an Estimated Conception date (useful context even if you dated from LMP or ultrasound rather than a known conception date), and the Next Milestone, flagging an upcoming point of interest in the pregnancy timeline.
A note on accuracy
These dates are estimates. As the calculator’s own note states, only about 1 in 20 babies arrives on the due date itself, and ultrasound dating from your healthcare provider takes precedence over any of these calculation methods if the two disagree. Always confirm dates and care decisions with your midwife or doctor rather than relying on this or any calculator alone.
Frequently asked questions
Which dating method is most accurate?
A first-trimester ultrasound is generally considered the most accurate dating method, since it measures the fetus directly rather than estimating from cycle assumptions. LMP dating is reliable when your cycles are regular and your last period date is remembered accurately, but ultrasound dating from your provider should take precedence if the two methods disagree.
Why does the calculator ask for embryo type in IVF dating?
A day-5 embryo is more developmentally advanced at transfer than a day-3 embryo, so the calculator adjusts its dating math based on which type you select, ensuring the calculated gestational age accounts for how far along the embryo already was at the moment of transfer.
Can I calculate gestational age for a date other than today?
Yes. The “Calculate as of” field defaults to today but can be set to any other date, useful for checking what your gestational age was (or will be) on a specific past or future date, such as an upcoming appointment.
