From the Guidelines
To calculate your last menstrual period (LMP), count back to the first day of your most recent period, which is the day when bleeding began, not when it ended. This method is widely used for determining gestational age, as stated in the study by 1. If you're tracking your periods on a calendar or app, simply look for the most recent start date. If you don't remember exactly, try to recall the week it occurred and estimate as accurately as possible.
The LMP is important because healthcare providers use it to estimate your due date if you're pregnant (by adding 280 days or 40 weeks) and to track your menstrual cycle regularity. For accuracy, it's helpful to maintain a period tracking system—either through a smartphone app, calendar, or journal—where you note the start date of each period. This practice not only helps you calculate your LMP when needed but also allows you to monitor your cycle length and identify any irregularities that might require medical attention.
Some key points to consider when calculating LMP include:
- The first day of your period is the day when bleeding began, not when it ended
- Maintaining a period tracking system can help with accuracy
- A typical menstrual cycle ranges from 21-35 days, though individual patterns vary
- LMP is used to estimate due date and track menstrual cycle regularity, as noted in the study by 1. However, it's worth noting that ultrasound measurement of crown-rump length (CRL) in early pregnancy provides an accurate estimate of gestational age, as stated in the study by 1.
From the Research
Calculating Last Menstrual Period (LMP)
To calculate the last menstrual period (LMP), several methods can be used, including:
- Using the first day of the last menstrual period as a reference point 2, 3, 4
- Utilizing ultrasound scans to estimate gestational age, which can be more accurate than LMP-based estimates 2, 3, 5, 6
- Combining LMP and ultrasound scans for a more accurate estimation of gestational age 4, 6
Factors Affecting LMP Calculation
Several factors can affect the accuracy of LMP calculations, including:
- Maternal characteristics, such as age, body weight, and ethnicity 2, 5
- Infant characteristics, such as birth weight 2, 5
- Digit preference in reporting LMP dates 4
- Systematic tendencies to overstate the duration of gestation 4
Comparison of LMP and Ultrasound-Based Estimates
Studies have compared LMP-based and ultrasound-based estimates of gestational age, with results showing that:
- Ultrasound-based estimates are generally more accurate than LMP-based estimates 2, 3, 5, 6
- LMP-based estimates tend to overestimate gestational age, leading to more post-term births being classified as such 2, 4, 5
- The combination of LMP and ultrasound scans can provide a more accurate estimation of gestational age 4, 6
Ultrasound-Based Estimation Methods
Different ultrasound-based estimation methods have been compared, including: