Free Tools
Due Date Calculator
Estimate your delivery date from your last period, conception date, or IVF transfer.
Want this in your pocket? Get the iOS app:
How this works
Pick the date you know best — most people start with the first day of their last menstrual period (LMP), which is also how clinicians date pregnancies. If you tracked ovulation or know your conception date, that's even more precise. For IVF pregnancies, use the embryo transfer date and tell us whether the embryo was transferred on day 3 or day 5.
The calculator runs the standard formula on whichever date you picked. Your estimated due date (EDD), current gestational age in weeks and days, current trimester, and days remaining all update instantly — no submit button, no page reload.
Most pregnancies end between 37 and 42 weeks, but only 39+0 to 40+6 counts as full term — 37 to 38+6 is early term (ACOG Committee Opinion 579). Only about 4% of babies arrive exactly on their estimated due date (Jukic 2013); the median spontaneous labour actually starts around 40 weeks and 5 days. Your provider may adjust your due date after a first-trimester ultrasound if measurements differ by more than seven days.
The math behind it
LMP mode uses Naegele's rule adjusted for cycle length — the method in ACOG Committee Opinion No. 700, and the one the ProbaBaby app applies. Dating LMP = the first day of your last period + (your cycle length − 28); EDD = dating LMP + 280 days (40 weeks). Naegele on its own assumes a regular 28-day cycle with ovulation on day 14, so a 35-day cycle lands a week later — the cycle-length slider does that for you instead of asking you to do it in your head.
Conception mode adds 266 days (38 weeks) to your stated conception date. Pregnancy duration is more accurately measured from conception than from LMP — clinicians use LMP as a proxy because it's easier to remember.
IVF mode subtracts the embryo's age at transfer from 266 days, then adds the remainder to your transfer date. For a day-3 transfer that's transfer + 263 days; for a day-5 (blastocyst) transfer it's transfer + 261 days. This is the ASRM-recommended method.
All three methods give an estimate — your healthcare provider's first-trimester ultrasound is the most accurate dating method available and may revise your EDD.
Frequently asked questions
Due date calculators are estimates. About 4% of babies arrive on the exact estimated date (Jukic 2013) — the median spontaneous labour actually starts around 40 weeks and 5 days — and most arrive within two weeks before or after. A first-trimester ultrasound (typically 8-13 weeks) is the most accurate method to date a pregnancy and may revise the due date provided here.
Tell the calculator. LMP mode has a cycle-length slider, and the dating start shifts by (cycle length − 28) days before Naegele's 280 days are added — the same ACOG Committee Opinion 700 adjustment the ProbaBaby app applies. A 35-day cycle therefore lands a week later than a 28-day one, automatically. If you know your conception date, that mode is more precise still, and a first-trimester ultrasound is more precise than either.
Use whichever you know better. LMP is more commonly remembered and is the dating standard for obstetric care. Conception date is more precise when you've been tracking ovulation, used at-home ovulation tests, or had a planned conception cycle.
For IVF pregnancies, the embryo's age at transfer matters because the developmental clock starts at fertilization, not at the transfer date. A day-3 embryo at transfer is already 3 days old, so EDD = transfer + 263 days. A day-5 blastocyst is 5 days old, so EDD = transfer + 261 days.
Yes. Your healthcare provider may revise your due date based on a first-trimester ultrasound (most reliable), the size of the gestational sac, or measurements like crown-rump length. If the ultrasound estimate differs from the LMP-based date by more than seven days, the ultrasound date is typically used.
Term is a ladder, not one range. Before 37 weeks is preterm; 37+0 to 38+6 is early term; 39+0 to 40+6 is full term; 41+0 to 41+6 is late term; 42+0 onward is post-term (ACOG Committee Opinion 579). Healthy births happen on every rung — but "full term" starts at 39 weeks, not 37, and that distinction matters if anyone offers you a date before then.