Pregnancy Due Date Calculator

Pregnancy Due Date & Gestational Age Calculator

This due date calculator estimates the estimated due date (EDD), the current gestational age and the conception date from whichever datum you have — last menstrual period (LMP), a first-trimester crown-rump length (CRL) or second-trimester biometry, a reported ultrasound gestational age, an IVF embryo transfer, a known conception date, or an established EDD. Unlike most online calculators, it also reconciles LMP against ultrasound using the ACOG / AIUM / SMFM Committee Opinion No. 700 redating rules and tells you which dating to keep, then lays out a dated calendar of the scans, screens and vaccinations tied to that pregnancy.

Whether you are a sonographer confirming an EDD at the dating scan, a clinician deciding whether a discrepant ultrasound should override the menstrual dates, or an expectant parent who simply wants an accurate due date, every method updates the same result instantly and produces a copyable dating summary. For embryo-by-embryo work in early pregnancy, pair it with our gestational age calculator, which dates from CRL and mean sac diameter.

Which Dating Method Is Most Accurate?

Accuracy falls as pregnancy advances, so the earliest reliable measurement wins. A first-trimester CRL dates a pregnancy to within roughly ±3–5 days and is the clinical gold standard between 7 and 14 weeks. The LMP is the least reliable input: it assumes a 28-day cycle with ovulation on day 14, which is wrong for most people, and recall is often imprecise. The calculator therefore lets you cycle-correct the LMP and, more importantly, redate to ultrasound when the discrepancy is large enough to matter.

When to Redate by Ultrasound (ACOG Committee Opinion No. 700)

When the menstrual dates and the ultrasound disagree, the EDD should be changed only if the difference exceeds a threshold that widens as gestation advances — early scans are precise, so a small discrepancy is meaningful, whereas late scans are imprecise and a larger gap is tolerated before redating. The Reconcile LMP vs ultrasound mode applies these thresholds for you and returns a clear keep-or-redate verdict with the final EDD.

Two rules catch people out. First, once an EDD is established by an early accurate scan, do not change it on a later scan — a third-trimester size discrepancy reflects fetal growth, not dating. Second, when the threshold is exceeded, redate to the ultrasound, not to a blend of the two. The calculator enforces both.

Dating by Ultrasound: CRL and the 84 mm Rule

In the first trimester, the crown-rump length gives the tightest dating. This tool uses the Robinson & Fleming (1975) equation — the standard adopted by NICE, RCOG and ISUOG — and shows the Tan (2023) estimate alongside for comparison. Once the CRL exceeds 84 mm (roughly 14 weeks), the fetus curls and straightens too variably for CRL to be reliable, so dating switches to the biometric parameters: biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC) and femur length (FL), combined with the Hadlock (1984) gestational-age equations. Enter any one or any combination and the calculator averages the available parameters.

For dating, Hadlock remains the most widely validated reference and tracks mean gestational age closely across the spectrum; alternative standards such as INTERGROWTH-21st and the WHO charts are primarily growth references and can differ from Hadlock by gestational age, particularly in the third trimester. Because dating should rest on the earliest scan, these later-pregnancy differences rarely change an EDD that was already set in the first trimester.

IVF and FET Due Dates: Day 3, Day 5 and Day 6

IVF dating is the most precise of all because the embryo’s age is known exactly. The rule is EDD = transfer date + (266 − embryo age in days): the embryo is already several days old at transfer, so that age is subtracted from the 266-day post-conception interval. A Day 3 cleavage-stage transfer adds 263 days; a Day 5 blastocyst adds 261 days; a Day 6 blastocyst adds 260 days. Fresh and frozen (FET) transfers use the identical rule — what matters is the embryo’s developmental day, not whether it was frozen, because a thawed Day 5 blastocyst is still five days old when transferred.

Your Pregnancy Milestone Calendar

From the EDD the calculator generates dated windows for the key events of pregnancy and marks each as completed, current or upcoming against today’s gestational age.

How to Use the Due Date Calculator

1. Choose the dating method that matches your most reliable datum — favour first-trimester ultrasound over LMP whenever you have it.
2. Enter the values. For LMP, add the cycle length to cycle-correct; for ultrasound, enter the CRL (or the biometry above 84 mm) with the scan date; for IVF, enter the transfer date and embryo day.
3. Reconcile if needed. When you have both an LMP and a scan, use the reconcile mode — the tool applies the Committee Opinion No. 700 thresholds and states whether to keep the LMP or redate.
4. Read, look up and copy. The EDD, current gestational age, conception date, trimester progress and milestone calendar update automatically; use the date look-up to find the gestational age on any past or future day, then copy the dating summary.

Frequently Asked Questions

When should you redate a pregnancy by ultrasound?

Redate when the difference between the LMP-derived EDD and the ultrasound EDD exceeds the Committee Opinion No. 700 threshold for the gestational age at the scan: more than 5 days at ≤ 8w6d, 7 days through 15w6d, 10 days through 21w6d, 14 days through 27w6d, and 21 days at or beyond 28 weeks. Within the threshold, keep the LMP dating. Once set by an early scan, the EDD is not changed by later scans.

Why is CRL more accurate than the last menstrual period?

The crown-rump length measures the embryo directly during a phase when growth is rapid and uniform between pregnancies, dating to within about 3–5 days. The LMP instead assumes a textbook 28-day cycle with ovulation on day 14 and depends on accurate recall — neither holds for many people, so LMP dating can be off by one to two weeks.

How is an IVF due date calculated?

EDD = transfer date + (266 − embryo age). A Day 3 transfer adds 263 days, a Day 5 blastocyst adds 261 days and a Day 6 blastocyst adds 260 days. Frozen embryo transfers use the same rule because the embryo’s developmental age, not the freezing, determines the dating.

At what CRL do you stop using crown-rump length for dating?

Above a CRL of 84 mm — about 14 weeks — CRL becomes unreliable because the fetus no longer lies in a consistent posture. Date instead from the biparietal diameter, head circumference, abdominal circumference and femur length using the Hadlock equations.

What does “suboptimally dated” mean?

ACOG applies the term to any pregnancy whose due date has not been confirmed or revised by ultrasound before 22 weeks 0 days. Such dating relies on the menstrual history alone and carries a wider error, which matters for decisions that hinge on gestational age, such as the timing of delivery for post-term pregnancy.

References

1. American College of Obstetricians and Gynecologists, AIUM, SMFM. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology. 2017;129(5):e150–e154.
2. Robinson HP, Fleming JEE. A critical evaluation of sonar crown-rump length measurements. British Journal of Obstetrics and Gynaecology. 1975;82(9):702–710.
3. Hadlock FP, Harrist RB, Shah YP, et al. Estimating fetal age: computer-assisted analysis of multiple fetal growth parameters. Radiology. 1984;152(2):497–501.
4. American Institute of Ultrasound in Medicine. AIUM Practice Parameter for the Performance of Standard Diagnostic Obstetric Ultrasound Examinations. 2018.
5. Papageorghiou AT, et al. International standards for fetal growth based on serial ultrasound measurements: the INTERGROWTH-21st Project. Lancet. 2014;384(9946):869–879.

This calculator implements published dating equations and the ACOG / AIUM / SMFM Committee Opinion No. 700 redating rules; in case of any discrepancy, the original publications and current clinical guidelines take precedence.

 

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