How many weeks pregnant would a woman be if her last menstrual period (LMP) started on August 13 and she hasn't had a period since, given today's date is October 11?

Pregnancy Dating Calculation Based on Last Menstrual Period

Based on the last menstrual period starting on August 13 and today being October 11, the woman would be approximately 8 weeks and 3 days pregnant.

Calculation Method

The calculation of gestational age follows standard obstetrical practice using the first day of the last menstrual period (LMP):

1. Starting date: August 13 (first day of LMP)
2. Current date: October 11
3. Time elapsed: 59 days (8 weeks and 3 days)

Understanding Pregnancy Dating

Pregnancy dating is conventionally calculated from the first day of the last menstrual period, not from the estimated date of conception 1. This method is endorsed by major medical societies including the Society of Radiologists in Ultrasound (SRU) and the American College of Radiology.

Key points about pregnancy dating:

• Gestational age is reported as number of weeks and number of days 2
• The first trimester is defined as gestational age ≤13 weeks 6 days 2
• Pregnancy dating should be established as early as possible for optimal prenatal care 3

Clinical Implications

At 8 weeks and 3 days:

• The embryo should be visible on transvaginal ultrasound
• Cardiac activity should be detectable (typically visible by 6 weeks) 2
• Crown-rump length measurement would be the most accurate method to confirm dating at this stage

Limitations of LMP-Based Dating

While LMP is the standard starting point for pregnancy dating, it has several limitations:

• Studies show that about 56% of women accurately recall their LMP date, with 74% recalling within 1 day 4
• Women with menstrual cycles longer than 28 days may have later ovulation, potentially leading to overestimation of gestational age 5
• Ultrasound dating in the first trimester (ideally between 7-14 weeks) provides more accurate dating with a standard deviation of ±7 days 1

Recommendations for Confirmation

For optimal pregnancy dating and care:

• An early ultrasound (ideally between 7-14 weeks) should be performed to confirm or adjust the estimated due date 1
• Crown-rump length measurement between 7-14 weeks provides accuracy of ±7 days 1
• Once established by early ultrasound, the estimated due date should rarely be changed 3

Conclusion

At 8 weeks and 3 days of pregnancy based on LMP, the woman is in the first trimester. This dating should be confirmed by an early ultrasound examination for optimal pregnancy management and accurate determination of the estimated due date.