What is the expected date of delivery (EDD) and when does the second trimester begin for a last menstrual period (LMP) of 12-08-2025?

Calculating EDD and Second Trimester Start Date for LMP 12-08-2025

For a last menstrual period (LMP) of December 8,2025, the expected date of delivery (EDD) is September 14,2026, and the second trimester begins at 14 weeks 0 days, which is March 16,2026.

Calculating Expected Date of Delivery (EDD)

The EDD calculation follows a standard approach based on the first day of the last menstrual period:

1. Start with LMP date: December 8,2025
2. Add 7 days: December 15,2025
3. Subtract 3 months: September 15,2025
4. Add 1 year: September 15,2026
5. Adjust by 1 day (since December has 31 days): September 14,2026

This calculation method is endorsed by major medical societies including the Society of Radiologists in Ultrasound (SRU) and the American College of Obstetricians and Gynecologists 1.

Determining Second Trimester Start

According to the Society of Radiologists in Ultrasound consensus recommendations:

• First trimester is defined as gestational age ≤ 13 weeks 6 days 2
• Therefore, second trimester begins at 14 weeks 0 days

For the LMP of December 8,2025:

• 14 weeks from LMP = March 16,2026

Important Clinical Considerations

Pregnancy Dating Accuracy

• First trimester ultrasound (ideally between 7-14 weeks) provides the most accurate dating, with a standard deviation of ±7 days 1
• Crown-rump length (CRL) measurement between 7-14 weeks provides an accuracy of ±7 days 1
• Studies show ultrasound dating is superior to LMP dating in predicting delivery date by at least 1.7 days 3

Common Pitfalls in EDD Calculation

1. Inaccurate LMP recall: Approximately 31.86% of women in some studies cannot accurately recall their LMP 4
2. Irregular menstrual cycles: When menstrual cycle differs from the standard 28 days, significant errors in EDD calculation can occur 5
3. Assuming rounded weeks: Gestational age should be reported as the number of weeks and days, not rounded weeks, to improve accuracy 1

Clinical Algorithm for Optimal Dating

1. If reliable LMP and regular 28-day cycles: Use LMP-based calculation
2. If ultrasound performed before 14 weeks: Use ultrasound dating if it differs from LMP dating by:
   • ≥3 days at <8+6 weeks
   • ≥5-7 days at 8+6 to 14 weeks
   • ≥7-10 days at 14-20 weeks 4
3. If irregular cycles or uncertain LMP: Rely on ultrasound dating

When ultrasound dating is used instead of LMP dating, studies show the number of post-term pregnancies decreases significantly (from 10.3% to 2.7%) 3, highlighting the clinical importance of accurate dating.