Due Date by Ultrasound
Enter your scan date & gestational age from the report
How to use: Find the gestational age (GA) on your ultrasound report: shown as weeks and days (e.g. "8w 3d"). Enter the scan date and that GA below.
The date the ultrasound was performed
wks
days
From your ultrasound report (e.g. 8 weeks 3 days)
Enter your ultrasound date and the gestational age from your scan report.
Estimated Due Date (EDD)
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Pregnancy progress
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Key Dates & Info
Ultrasound date—
GA at scan—
Current gestational age—
Estimated LMP—
Est. conception date—
Days until due date—
Estimated due date—
Trimesters
1st
Wk 1 to 13
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2nd
Wk 14 to 27
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3rd
Wk 28 to 40
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Ultrasound Dating: How It Works
To get a due date from an ultrasound, subtract the gestational age on the report from the scan date, then add 280 days. A scan on September 21, 2026 that measures 8 weeks 3 days points to an estimated last period of July 24, 2026 and a due date of April 30, 2027. Enter your scan date and gestational age above and the calculator does the counting.
Ultrasound dating works by measuring the baby's size and comparing it to expected measurements at different gestational ages. In the first trimester, the crown-rump length (CRL) is measured and provides the most accurate gestational age estimate: accurate to within 5 to 7 days when performed at 8 to 12 weeks. This is more accurate than LMP-based dating, which assumes a 28-day cycle and day-14 ovulation.
This calculator works backwards from the gestational age shown on your ultrasound report. It subtracts the GA from the scan date to determine the estimated LMP date, then adds 280 days (40 weeks) to calculate the EDD. The conception date is estimated as LMP plus 14 days (day of ovulation in a standard cycle).
When Is Ultrasound Most Accurate?
First trimester dating (8 to 12 weeks) is the most accurate, with a margin of plus or minus 5 days. Second trimester (14 to 22 weeks) has a margin of plus or minus 10 to 14 days. Third trimester scans are the least accurate for dating (margin of 3 to 4 weeks) because fetal size varies significantly between individuals. ACOG guidelines recommend using the earliest ultrasound for dating when there is a discrepancy with LMP.
Gestational Age vs Fetal Age
Gestational age (GA) is counted from the LMP, including the 2 weeks before conception. Fetal age is counted from conception and is approximately 2 weeks less than GA. When your report says "8 weeks," the baby is developmentally about 6 weeks old. All obstetric calculations, trimester definitions, and developmental milestones use gestational age as the standard.
What If Ultrasound Disagrees With LMP?
ACOG advises using the ultrasound date when it differs from LMP dating by more than 5 days before 9 weeks, 7 days from 9 to 15 weeks 6 days, 10 days from 16 to 21 weeks 6 days, or 14 days from 22 to 27 weeks 6 days. This is because the LMP method assumes regular 28-day cycles and day-14 ovulation, while ultrasound measures actual fetal size. Discrepancies are common with irregular cycles or uncertain LMP dates.
Multiple Ultrasounds
If you have had multiple ultrasounds, use the earliest one for due date calculation: it is the most accurate. Later scans may show different GA estimates due to normal variation in fetal growth rates, but the due date established by the first scan should not routinely be changed unless there is a significant clinical reason. Growth scans assess fetal wellbeing, not dating accuracy.
When the Ultrasound Date Replaces Your LMP Date
The American College of Obstetricians and Gynecologists (ACOG) sets out when a scan should override the due date from your last menstrual period (LMP). The earlier the scan, the smaller the difference needed, because early babies vary very little in size.
| Gestational age at scan | Measurement used | Use the ultrasound date if it differs by more than |
|---|---|---|
| Up to 8 weeks 6 days | Crown-rump length (CRL) | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | Crown-rump length (CRL) | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | Head, abdomen and femur (BPD, HC, AC, FL) | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | Head, abdomen and femur (BPD, HC, AC, FL) | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | Head, abdomen and femur (BPD, HC, AC, FL) | 14 days |
| 28 weeks 0 days or later | Head, abdomen and femur (BPD, HC, AC, FL) | 21 days |
Once a due date has been set from the earliest scan, ACOG says it should only be changed in rare cases. A later growth scan that shows a bigger or smaller baby is read as a growth finding, not as a new due date. If the first scan happens after 22 weeks with no earlier dating, the pregnancy is treated as less precisely dated.
Crown-Rump Length and Gestational Age
In the first trimester the sonographer measures the crown-rump length (CRL), the straight line from the top of the head to the bottom of the torso. The classic Robinson and Fleming formula converts it to gestational age: days = 8.052 × √CRL in mm + 23.73. Ultrasound machines may use this or a newer chart such as Hadlock or INTERGROWTH-21st, so your report can differ by a day or so. Enter the gestational age printed on your report, not a value from this table.
| CRL | Gestational age | CRL | Gestational age |
|---|---|---|---|
| 5 mm | 6 weeks 0 days | 40 mm | 10 weeks 5 days |
| 10 mm | 7 weeks 0 days | 45 mm | 11 weeks 1 day |
| 15 mm | 7 weeks 6 days | 50 mm | 11 weeks 4 days |
| 20 mm | 8 weeks 4 days | 60 mm | 12 weeks 2 days |
| 25 mm | 9 weeks 1 day | 70 mm | 13 weeks 0 days |
| 30 mm | 9 weeks 5 days | 84 mm | 14 weeks 0 days |
A CRL of 84 mm corresponds to about 14 weeks. Beyond that, dating switches to head, abdomen and thigh bone measurements.
Worked Example: Scan vs Last Period
- Your dating scan on September 21, 2026 reports 8 weeks 3 days, which is 59 days.
- September 21, 2026 minus 59 days gives an estimated LMP of July 24, 2026.
- July 24, 2026 plus 280 days gives a due date of April 30, 2027. Estimated conception is 14 days after the LMP date, around August 7, 2026.
- Now compare with your period. If your last period actually started on July 20, 2026, the LMP due date is April 26, 2027. The difference is 4 days.
- The scan was before 9 weeks, where the threshold is more than 5 days. A 4-day gap is within it, so your provider would normally keep the LMP date of April 26. A gap of 6 days or more would switch the due date to the scan.
Working from your last period instead? Use the pregnancy due date calculator, and see the conception calculator to work back to the likely conception date.
Method and sources. Estimated LMP = scan date minus gestational age at the scan; due date = estimated LMP + 280 days; estimated conception = estimated LMP + 14 days. Redating thresholds: ACOG Committee Opinion 700, Methods for Estimating the Due Date (developed with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine). CRL table computed with the Robinson and Fleming (1975) formula. Term definitions: ACOG Committee Opinion 579. Results are estimates; your obstetrician or midwife sets the official due date.
This calculator uses the gestational age from your ultrasound report to estimate your due date. Results are estimates. Your healthcare provider may use additional clinical information to establish or adjust your EDD. Always consult your obstetrician or midwife for medical guidance during pregnancy.
Ultrasound Due Date Questions
The due date is calculated by subtracting the gestational age at the time of the scan from the scan date to find the estimated LMP (last menstrual period) date, then adding 280 days (40 weeks) to that LMP date. For example: if your scan on March 1st shows 8 weeks 0 days of gestational age, the estimated LMP is January 4th (56 days before March 1st), and the EDD is October 11th (280 days after January 4th). This calculator performs this arithmetic automatically using the scan date and GA from your report.
In the first trimester (especially 8 to 12 weeks), fetal size varies very little between individuals of the same gestational age. The crown-rump length (CRL) measurement at this stage has a very tight standard deviation, giving a margin of plus or minus 5 days. As pregnancy progresses, natural variation in fetal growth increases dramatically. By the third trimester, two babies of the same gestational age might differ in weight by a kilogram or more, making size-based dating unreliable. This is why ACOG recommends establishing the EDD from the earliest ultrasound and not routinely changing it based on later scans.
In most cases, the ultrasound date is more reliable, particularly if the scan was performed in the first trimester. LMP dating assumes you have a 28-day cycle with ovulation on day 14, which is not true for everyone. Common reasons for discrepancy: irregular cycles, late ovulation, uncertain LMP date, or early implantation. ACOG recommends using the ultrasound EDD when it differs from the LMP date by more than 5 days before 9 weeks, more than 7 days from 9 weeks to 15 weeks 6 days, more than 10 days from 16 weeks to 21 weeks 6 days, and more than 14 days from 22 weeks to 27 weeks 6 days. From 28 weeks 0 days, ultrasound dating is the least reliable, accurate only to within about 21 to 30 days, and ACOG warns against redating on a third-trimester scan alone, because a small baby may be growth restricted rather than younger than thought. Discuss any significant discrepancy with your doctor or midwife.
Gestational age on an ultrasound report represents how far along the pregnancy is, counted from the first day of the last menstrual period (LMP), not from conception. The sonographer measures the baby (typically crown-rump length in the first trimester, or head circumference, abdominal circumference, and femur length later) and compares it to population charts to determine which gestational age that size corresponds to. GA is expressed in weeks and days (e.g., 10w 4d). This is approximately 2 weeks more than the baby's actual age from conception.
Officially, the EDD established by the first ultrasound (especially if done in the first trimester) should not routinely be changed based on subsequent growth scans. Later scans showing a "smaller" or "larger" baby reflect normal variation in fetal growth, not necessarily a different gestational age. ACOG says an EDD set from the earliest ultrasound should be changed only in rare circumstances, and that decision rests with your provider. Growth scans (typically done from 28 weeks) are designed to assess fetal wellbeing and growth rate, not to redetermine the due date.
Pregnancy is divided into three trimesters based on gestational age: First trimester: weeks 1 to 13 (conception through early organogenesis; highest miscarriage risk; most accurate for dating). Second trimester: weeks 14 to 27 (often called the "comfortable" trimester; anomaly scan at 18 to 22 weeks; fetal movement begins). Third trimester: weeks 28 to 40+ (rapid fetal growth; preparation for birth; growth scans; GBS testing; birth plan discussions). Full term is 39 weeks 0 days to 40 weeks 6 days, with 37 to 38 weeks as early term, 41 weeks as late term and 42 weeks or more as post-term.
Different measurements are used depending on the stage of pregnancy. In the first trimester (6 to 13 weeks): crown-rump length (CRL) is the primary measurement, the straight-line distance from top of head to bottom of torso. In the second trimester (14 to 28 weeks): biparietal diameter (BPD, head width), head circumference (HC), abdominal circumference (AC), and femur length (FL) are all used and combined for an estimate. Third trimester (28+ weeks): the same four measurements are used, but their accuracy for dating decreases significantly. The ultrasound software automatically calculates the gestational age from these measurements using established population charts.
No. The EDD is a statistical estimate of when delivery is most likely: it is the midpoint of a normal distribution, not a fixed date. Only about 4-5% of babies are born on their exact due date. A pregnancy is considered full term from 39 to 40 weeks, early term at 37 to 38 weeks, late term at 41 weeks, and post-term at 42+ weeks. Many providers will discuss induction options around 41 to 42 weeks for post-term pregnancies. The EDD is best understood as the center of a 4-5 week window during which birth is expected.
A dating scan (also called a viability scan or booking scan) is typically performed at 6 to 10 weeks. Its primary purposes are to confirm a viable intrauterine pregnancy, establish or confirm the EDD, detect multiple pregnancies, and assess early fetal anatomy. It is the most accurate time for establishing gestational age. An anomaly scan (also called a mid-pregnancy scan, anatomy scan, or 20-week scan) is performed at 18 to 22 weeks. It is a detailed examination of fetal anatomy, checking organ development and structure for anomalies. It can still provide a GA estimate but is less accurate for dating than the first trimester scan. The NT (nuchal translucency) scan at 11 to 14 weeks screens for chromosomal conditions and can also confirm or adjust the EDD.
This calculator uses the same underlying formula that obstetricians and midwives use: LMP date plus 280 days for the EDD, derived by working backwards from the ultrasound scan date and gestational age. The arithmetic is identical. The difference is that your healthcare provider has access to your full clinical history, multiple measurements, your actual scan images, and clinical judgment about which result to use when there are discrepancies. This calculator is accurate for the pure date arithmetic but cannot replicate the clinical decision-making that goes into establishing and potentially revising your official EDD. Use it as a reference tool, not a medical determination.
CRL is the length of the embryo or fetus from the top of the head to the bottom of the torso, measured in millimeters. It is the standard way to date a pregnancy up to 13 weeks 6 days because babies of the same age are almost the same size at that point. A CRL of 10 mm is about 7 weeks 0 days and 45 mm is about 11 weeks 1 day using the Robinson and Fleming formula. Your report will already show the gestational age that goes with the measurement.
Only if it is your first scan. If you already had a first trimester dating scan, the due date from that scan stands. If the 20-week scan is the first ultrasound of the pregnancy, ACOG advises switching to the scan date when it differs from your LMP date by more than 10 days (the rule for 16 weeks 0 days to 21 weeks 6 days). Smaller differences are normal variation in growth.