Tool 01
Due date
calculator
Four ways in, one answer. Whichever you use, the result is the same 280 day count, so the modes can never disagree with each other.
Estimated due date
- You are
- Days pregnant
- Days to go
- Trimester
Also worked out
- Conception around
- Day of the week
- Star sign
- Birthstone
Your baby right now
Every date that hangs off it
Your timeline
Filled circles are behind you. The coral one is open now. Windows are windows for a reason: the nuchal measurement is only valid inside eight days, and the strep swab has to land in the fortnight before birth.
Keep the timeline
Get each date the week before it matters
Nobody remembers that the NT scan window is eight days long. One email a week, timed off the date you just calculated.
How each mode works
From your last period
The due date is 280 days from the first day of your last period. That is 40 weeks, and it is the count every practice uses. It assumes a 28 day cycle with ovulation on day 14, which is why this calculator asks for your cycle length: if you ovulate on day 21 you conceived a week later than the standard assumption and your due date is a week later too.
The odd part, and it catches everyone, is that the count starts about two weeks before conception. At the moment you conceive you are already considered four weeks pregnant by the end of that fortnight. The convention exists because the first day of a period is a date most people can name and the day of ovulation is not.
From a known conception date
266 days from conception. If you tracked ovulation or know the single day it could have happened, this is tighter than working back from a period.
From an IVF transfer
The most precise of the four, because the age of the embryo is known exactly. A day 5 blastocyst is due 261 days after transfer, a day 3 embryo 263 days after, and an egg retrieval or IUI 266 days after. All three are the same arithmetic: 280 days minus 14 minus the age of the embryo on the day it went in.
From a dating ultrasound
A scan measures the baby and reports a gestational age, which fixes the whole timeline from that day. In the first trimester the scan is more accurate than a date worked out from a period, and where the two disagree by more than about a week your practice will re-date you from the scan. Enter the date of the scan and the age it gave.
How accurate is a due date
About 4 babies in 100 arrive on the due date itself. Roughly 80 percent arrive in the two weeks either side of it, which is why the term window runs from 37 weeks to 42. A due date is a midpoint, not a deadline, and treating it as a deadline is the source of most of the anxiety in the last month.
What this calculator does not do
It does not know anything about you. It cannot tell you whether a pregnancy is healthy, whether a date should be revised, or what any symptom means. It is arithmetic on a calendar, done carefully. Everything else is a conversation with your midwife or doctor.
If the date this returns is very different from the one you were given, the one you were given is probably right and probably came from a scan. Use the ultrasound mode above to match it.