How Pregnancy Weeks Are Calculated: LMP, Ultrasound, and Due Date Explained

Pregnancy dating provides the foundation for monitoring fetal development, planning prenatal care, and estimating the expected date of delivery (EDD). Clinically, the duration of pregnancy is measured in completed weeks from the first day of the last menstrual period (LMP). Although conception usually occurs about two weeks later, this standardized method ensures uniformity in clinical assessment and obstetric reporting.

Last Menstrual Period (LMP): The Starting Point

The LMP marks the first day of bleeding in the last normal menstrual cycle before conception. Gestational age is counted from this day, even though fertilization typically occurs about 14 days later in a 28-day cycle.

This method assumes regular menstrual cycles and ovulation on day 14. However, late ovulation, irregular cycles, stress, hormonal changes, or breastfeeding can delay ovulation, making the pregnancy appear younger on ultrasound than predicted by the LMP.

Gestational Age vs. Embryonic Age

• Gestational Age (GA): Time elapsed since the first day of the LMP.
• Embryonic Age: Actual biological age of the embryo, measured from conception, usually about two weeks shorter than the gestational age.

For example, a pregnancy described as 8 weeks gestational age equals a 6-week embryo. Clinicians use gestational age for standardization, while embryonic age is used in embryological studies and assisted reproduction.

Methods of Estimating Gestational Age

1. Calculation Based on LMP

This is the traditional method used when menstrual cycles are regular. Pregnancy duration is simply counted in completed weeks from the LMP.

2. Estimation by Ultrasound

Ultrasound is the most accurate method for dating pregnancy, particularly during the first trimester. The Crown–Rump Length (CRL), the distance from the fetal head to buttocks, reflects true gestational age.

• At 8 weeks, CRL ≈ 2.0 cm
• Accuracy: ±5 days

If the LMP-based and ultrasound-based gestational ages differ by more than 7 days during the first trimester, the ultrasound age is preferred for future pregnancy monitoring and EDD determination.

Expected Date of Delivery (EDD)

The EDD indicates the probable date of childbirth, typically at 40 weeks of gestation. Although only about 5% of women deliver on this exact date, it remains a key reference for clinical care.

Naegele’s Rule for EDD Calculation

Example: If LMP = 10 January 2025 → EDD = 17 October 2025.

Adjustments should be made if the menstrual cycle is shorter or longer than 28 days.

EDD by Ultrasound

When early ultrasound is available, EDD can be calculated from the ultrasound-derived gestational age. For instance, a CRL corresponding to 8 weeks on 20 March 2025 indicates an EDD around 15 November 2025 (8 + 32 weeks).

Why LMP and Ultrasound Dates May Differ

A difference between the two methods is common and usually harmless.

Reasons include:

• Delayed or early ovulation
• Irregular menstrual cycles
• Late implantation
• Miscalculated or uncertain LMP
• Biological variation in early embryonic growth

Discrepancies of up to one week are considered normal and do not imply abnormal development.

Importance of Accurate Gestational Dating

Precise gestational dating is critical for:

• Monitoring fetal growth and identifying growth restriction or macrosomia
• Scheduling time-sensitive prenatal tests (NT scan, anomaly scan, etc.)
• Assessing preterm and post-term pregnancies
• Planning delivery or induction at the correct time
• Confirming pregnancy viability through correlation of fetal size and heartbeat with gestational age

Typical Gestational Milestones

Pregnancy Timeline and Trimesters

• First trimester: 0–13 weeks — rapid organ development and hormone surge
• Second trimester: 14–27 weeks — fetal growth and maternal adaptation
• Third trimester: 28–40 weeks — maturation and preparation for birth

Pregnancies between 37 and 42 weeks are considered term. Deliveries before 37 weeks are preterm, while those beyond 42 weeks are post-term.

Integrating LMP and Ultrasound Data

The best approach combines both methods:

1. Use LMP as the initial guide.
2. Verify and correct with first-trimester ultrasound.
3. Once confirmed, use the ultrasound-based gestational age and EDD consistently for all future evaluations.

This approach eliminates confusion and ensures reliable monitoring throughout pregnancy.

Summary

• LMP serves as the starting reference for pregnancy dating.
• Gestational age is counted from the LMP; embryonic age begins about two weeks later.
• EDD is calculated using Naegele’s rule or refined via early ultrasound (CRL method).
• When discrepancies occur, ultrasound dating is more accurate.
• Minor differences between LMP and ultrasound results are common and reflect natural reproductive variation.
• Accurate dating enables optimal prenatal care, growth monitoring, and delivery planning.

Conclusion

Pregnancy dating unites clinical observation with biological precision. The LMP provides a convenient chronological framework, while early ultrasound ensures scientific accuracy in determining gestational age and EDD. A slight difference between menstrual and ultrasound ages is normal and usually results from variations in ovulation or implantation timing. Understanding these principles allows expectant parents and clinicians to interpret pregnancy milestones correctly and ensure a healthy outcome for both mother and child.