When does the second trimester begin if the Last Menstrual Period (LMP) is August 12th, 2025?

Second Trimester Beginning Date Based on LMP August 12,2025

The second trimester will begin on November 26,2025, which is 14 weeks and 0 days from the last menstrual period (LMP) of August 12,2025. 1, 2

Calculation Method

According to the Society of Radiologists in Ultrasound (SRU) consensus guidelines, pregnancy dating is calculated using the following principles:

1. Gestational age (GA) is reported as number of weeks and days
2. GA is calculated using the first day of the last menstrual period (LMP) and/or an early-dating ultrasound examination
3. The first trimester is defined as GA ≤ 13 weeks 6 days 1

Therefore, to determine when the second trimester begins:

• LMP: August 12,2025
• First trimester ends: 13 weeks 6 days from LMP
• Second trimester begins: 14 weeks 0 days from LMP

Step-by-Step Calculation

1. Start with LMP date: August 12,2025
2. Add 13 weeks and 6 days: November 25,2025
3. The next day (14 weeks 0 days): November 26,2025

Important Considerations

Accuracy of LMP Dating

While LMP is commonly used for initial dating, it's important to note that:

• First trimester ultrasound (ideally between 7-14 weeks) provides the most accurate dating, with a standard deviation of ±7 days 2
• LMP-based estimates are on average 0.8 days longer than ultrasound estimates 3
• The biologic variation of the time from LMP to actual pregnancy start is approximately 7.0 days (standard deviation) 4

Clinical Implications

• For optimal prenatal care, an early ultrasound is recommended to confirm dating, especially if there are concerns about LMP reliability
• Accurate dating is crucial for:
  • Timing of prenatal screening tests
  • Assessment of fetal growth
  • Appropriate timing of delivery
  • Reducing misclassification of preterm and post-term pregnancies 2, 5

Common Pitfalls to Avoid

1. Relying solely on LMP without ultrasound confirmation: Studies show ultrasound is superior to LMP in predicting delivery date by at least 1.7 days 5
2. Rounding to completed weeks: Always report gestational age as weeks and days for greater accuracy 2
3. Assuming LMP is always reliable: LMP may be unreliable due to irregular cycles, bleeding in early pregnancy, or recall bias 3

When both LMP and ultrasound dating are available, ultrasound dating in the first trimester generally takes precedence, especially when there is a significant discrepancy between the two methods 6.