Due Date by Ultrasound Calculator

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 8–12 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. Clinical guidelines (ACOG, NICE) recommend using the ultrasound EDD when it differs from LMP by more than 5 days in the first trimester, or more than 10 days in the second trimester. 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. However, there are exceptions: if a dating scan was done very early (before 8 weeks) when accuracy is lower, or if there is a large and clinically significant discrepancy, your provider may revise the EDD. 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 1–13 (conception through early organogenesis; highest miscarriage risk; most accurate for dating). Second trimester: weeks 14–27 (often called the "comfortable" trimester; anomaly scan at 18–22 weeks; fetal movement begins). Third trimester: weeks 28–40+ (rapid fetal growth; preparation for birth; growth scans; GBS testing; birth plan discussions). Term pregnancy is defined as 39–40 weeks (full term), with 37–38 weeks as early term and 41–42 weeks as late term.

Different measurements are used depending on the stage of pregnancy. In the first trimester (6–13 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 (14–28 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. About 70% are born within 10 days of the EDD (before or after), and approximately 90% within 2 weeks. A pregnancy is considered full term from 39–40 weeks, early term at 37–38 weeks, late term at 41 weeks, and post-term at 42+ weeks. Many providers will discuss induction options around 41–42 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 6–10 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 18–22 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 11–14 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.