Due Date by Ultrasound — Most Accurate Dating Method

Why Due Dates Are Always Estimates

A due date is a population-level reference point, not a scheduled arrival. Only about 5% of birthing people deliver spontaneously on their calculated due date, and roughly 80% deliver within a ±2-week window around it. Variation comes from normal biological differences in implantation timing, menstrual-cycle length, fetal growth patterns, and individual factors we do not fully measure with simple calculators. Your prenatal care team will adjust your official dating based on first-trimester ultrasound crown-rump measurement when needed, which is the most accurate reference window we have.

Method 1 — Naegele Rule (LMP + 280 Days)

Method 2 — Conception Date

If you know the approximate date of ovulation, intercourse, or IUI or IVF fertilization, a conception-date estimate adds 266 days (38 weeks, the average duration of human gestation from fertilization to term) instead of 280. This removes the two-week LMP-to-conception offset, so the raw calculation does not depend on cycle length. Keep in mind that sperm can survive inside the female reproductive tract for several days, so if you are estimating from intercourse the actual fertilization window can span multiple days.

Method 3 — IVF Transfer

Cycle Length and LMP Dating

Naegele rule assumes a 28-day cycle. For people with consistently longer or shorter cycles, a simple linear adjustment (shifting the estimate by [cycle length − 28] days) can be applied. However, because luteal-phase length is relatively stable across cycle lengths (about 14 days) while follicular phase varies, adjusting LMP-based estimates purely for cycle length is an approximation at best. For irregular cycles, conception-date or first-trimester-ultrasound dating is generally more reliable.

Trimester Split

Three standard trimesters are used for reference and prenatal-care scheduling. The first trimester runs from LMP through week 12 + 6 days and covers the period of highest sensitivity for embryological development. The second trimester spans week 13 through week 27 + 6 days and is the typical window for anatomical screening, prenatal sugar screening, and fetal-growth surveys. The third trimester runs from week 28 through delivery, with the viability reference point commonly placed at 24 weeks and the term reference window at 37–42 weeks.

Why Ultrasound Is the Most Accurate Dating Reference

A first-trimester ultrasound measuring the crown-rump length (CRL) — the distance from the top of the fetal head to the bottom of the buttocks — is the most accurate reference for pregnancy dating. At 8 to 12 weeks, fetuses grow at a very similar rate, so the CRL measurement places the gestational age within a margin of about 5 to 7 days. This is markedly tighter than the LMP-based Naegele estimate, which has a population margin of roughly plus or minus 2 weeks because cycle lengths, ovulation timing, and LMP recall all vary. For this reason, when the ultrasound-based dating differs from the LMP-based dating by more than 5 to 7 days in the first trimester, the care team will typically use the ultrasound-adjusted dating for all subsequent scheduling.

Crown-Rump Length and the Hadlock Reference

The CRL measurement is converted to a gestational age using published reference charts. The Robinson and Hadlock charts are the most widely used; both map a CRL in millimeters to a gestational age in weeks and days. For example, a CRL of about 30 mm corresponds to roughly 9 weeks and 5 days; a CRL of about 60 mm corresponds to roughly 12 weeks and 3 days. Your prenatal care team records the CRL measurement and the derived gestational age in your prenatal chart. If the ultrasound-derived gestational age differs from the LMP-derived gestational age by more than the first-trimester margin (about 5 to 7 days), the ultrasound dating supersedes the LMP dating for all subsequent scheduling.

First vs Second Trimester Ultrasound Dating

First-trimester ultrasounds (8 to 12 weeks) using CRL are the most accurate dating reference. Second-trimester ultrasounds (13 to 22 weeks) are less accurate for dating because fetal size variation increases — the margin widens to about 10 to 14 days. By the third trimester, ultrasound dating is even less precise (margin of about 2 to 3 weeks) because individual growth differences dominate. For this reason, dating is ideally confirmed at the first-trimester ultrasound and is not usually re-dated later in the pregnancy, even if later ultrasounds suggest a different fetal size. If you missed the first-trimester window, a second-trimester ultrasound still provides a useful dating reference, just with a wider margin.

Ultrasound vs LMP — Which to Use?

Use LMP-based dating (Naegele rule with cycle length adjustment) as the initial reference until the first prenatal ultrasound. If your cycles are regular and match the 28-day pattern, the LMP and first-trimester ultrasound dates usually agree within a few days, and either can be used. If your cycles are irregular, if LMP recall is uncertain, or if the LMP and ultrasound dates differ by more than 5 to 7 days in the first trimester, the ultrasound date supersedes the LMP date for all subsequent scheduling. After the first trimester, the gestational age is considered fixed and is not usually re-dated, even if later ultrasounds suggest a different fetal size — instead, the size relative to the established date is used to assess growth.

When to Choose Ultrasound Dating

Ultrasound dating is the preferred reference whenever LMP is uncertain, cycles are irregular, recent hormonal contraception has altered bleed patterns, implantation bleeding was mistaken for a period, or LMP and ultrasound dating disagree by more than the first-trimester margin. Ultrasound dating is also the reference for IVF and IUI pregnancies — although the IVF transfer date is documented to the day, the first-trimester ultrasound confirms that growth is on track. The dating ultrasound is typically scheduled between 8 and 12 weeks and is often combined with first-trimester screening.

Practical Reference Tips

• Schedule the dating ultrasound for 8 to 12 weeks, when CRL measurement is most accurate (margin about 5 to 7 days).
• If the ultrasound dating differs from your LMP dating by more than 5 to 7 days in the first trimester, the care team will use the ultrasound date for all subsequent scheduling.
• After the first trimester, gestational age is considered fixed and is not usually re-dated. Later ultrasounds assess growth against the established date.
• If you missed the first-trimester window, a second-trimester ultrasound still provides a useful dating reference, just with a wider margin (about 10 to 14 days).
• Bring any LMP, ovulation tracking, or IVF records to the ultrasound — the care team uses all available references to confirm dating.