Most pregnancies get two due dates. The first comes from your last menstrual period and is often calculated at home with a pregnancy calculator. The second comes from the dating ultrasound done in early pregnancy. When these two dates don’t match, expectant parents may be confused.
Understanding how each method works, and when one is more reliable than the other, helps families approach antenatal visits with clarity. It also matters because the due date shapes almost every pregnancy milestone, from scan scheduling to delivery planning.
How Last Menstrual Period (LMP) Dating Works
LMP dating counts 40 weeks (280 days) from the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation on day 14. This is the method used by most online pregnancy calculator tools and is the first estimate most women receive.
LMP dating works well when:
- Menstrual cycles are consistently 28 days long
- The date of the last period is remembered accurately
- Ovulation happened on day 14 of the cycle
- There was no recent use of hormonal contraception
- There was no breastfeeding affecting cycle regularity
LMP dating is simple and non-invasive. Its weakness is that it falls apart quickly when cycles are irregular, ovulation is delayed, or the exact date of the last period is uncertain.
How Ultrasound Dating Works
Ultrasound dating uses the measurement of the growing embryo or foetus to estimate gestational age. In early pregnancy, between 7 and 13 weeks, the crown-rump length (CRL) is the most accurate measurement. After 14 weeks, clinicians use measurements such as biparietal diameter, head circumference, and femur length.
Ultrasound dating is most accurate in the first trimester. A dating scan done between 7 and 13 weeks is accurate to within 5 to 7 days. Accuracy decreases as pregnancy progresses because babies grow at different rates in later months.
When Do the Two Dates Disagree?
Disagreement between LMP and ultrasound dating is common. Reasons include:
- Irregular menstrual cycles longer or shorter than 28 days
- Late or early ovulation in the cycle of conception
- Uncertain recall of the last period date
- Recent discontinuation of hormonal contraception
- Breastfeeding or postpartum hormonal shifts
- Pregnancy conceived through IVF, which has its own precise dating
When the two estimates differ, obstetric guidelines offer clear direction on which to use.
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Which Date Does the Obstetric Team Use?
International guidelines, including those followed in India, recommend ultrasound dating as the gold standard when the two estimates differ by more than 7 days in the first trimester. If the ultrasound estimate differs by less than 7 days, keep the LMP date. The ultrasound-based due date then becomes the official date, and all subsequent pregnancy milestones are calculated from it.
This matters because an inaccurate due date affects when screening tests are offered, how foetal growth is assessed, and when delivery is planned. Using the more accurate date reduces unnecessary interventions such as induction for suspected post-term pregnancy that is actually not post-term at all.
Why Early Dating Scans Matter
A dating scan between 7 and 13 weeks is one of the most important early pregnancy appointments. Benefits include:
- Confirmation of pregnancy viability
- Accurate gestational age calculation
- Detection of multiple pregnancy
- Identification of pregnancy location (ruling out ectopic)
- Baseline measurement for first-trimester screening
Booking the first antenatal visit early in the first trimester allows this scan to happen within the window of maximum accuracy.
Special Cases: IVF Pregnancies
IVF pregnancies are dated differently. The embryo transfer date is known with certainty, and gestational age is calculated from the embryo age plus 14 days. This dating is more precise than any ultrasound estimate. Expectant parents who conceive through IVF should mention this clearly at the first antenatal visit so that the obstetric team uses the IVF-based date rather than defaulting to LMP or ultrasound.
Trusted Antenatal Dating Across India
Established maternity hospitals such as BirthRight by Rainbow Hospitals follow international guidelines on pregnancy dating and offer early first-trimester scans at every centre. As a trusted Maternity Hospital in India, the group combines modern ultrasound diagnostics with experienced obstetric teams to confirm pregnancy, establish accurate dating, and plan antenatal care from the earliest visit onwards.
Conclusion
The due date shapes the entire pregnancy journey. LMP dating offers a reasonable first estimate, but ultrasound dating in the first trimester is the more accurate tool when the two disagree. Book an early dating scan, bring any IVF treatment records if applicable, and trust the obstetric team to use the method that fits your situation. A well-established due date is the foundation of safe, well-planned antenatal care.
Medical Disclaimer
The information given in this content is meant for educational and informational purposes only. In case of any health issues, please consult your doctor.