When discovering a pregnancy, the first question arises within minutes: where exactly are we? Even before the first medical appointment, the date of the last menstrual period provides a reliable reference point to date the beginning of the pregnancy and estimate the due date. However, it is essential to understand what this calculation truly measures and where it reaches its limits.
Why the calculation is based on the last menstrual period and not on conception
One might think that counting from the day of conception would be more logical. In practice, most women know the date of their last menstrual period with much more certainty than the exact day of fertilization.
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The calculation therefore starts from the first day of the last menstrual period, even if the pregnancy has not yet begun at that time. Ovulation generally occurs around the fourteenth day of the cycle, but this timeframe varies from woman to woman. By choosing a stable starting point, a common reference framework is established that is used by all healthcare professionals.
This reference can be refined using the pregnancy calculator with Babys Corner, which details the steps to find this date and deduce the estimated due date.
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Weeks of amenorrhea or weeks of pregnancy: what each count measures
The confusion between these two counting systems arises with every pregnancy. It is not trivial, as it can shift the perception of progress by two weeks.
The count in weeks of amenorrhea
Amenorrhea refers to the absence of menstruation. We count in weeks of amenorrhea (WA) from the first day of the last menstrual period. This is the system used in medical reports, ultrasound prescriptions, and follow-up calendars.
This count therefore includes the two weeks before conception. When reading “12 WA” on a report, the baby is approximately 10 weeks of actual development.
The count in weeks of pregnancy
Weeks of pregnancy (WP) start from the estimated day of conception, which is about two weeks after the start of the last menstrual period. This method of calculation corresponds more closely to embryonic development, but it is less commonly used in the French medical environment.
To switch from one system to the other, two weeks are subtracted from WA. For example, 20 WA corresponds to 18 WP.
- Ultrasounds, blood tests, and screening follow-ups are dated in weeks of amenorrhea.
- Public applications sometimes use weeks of pregnancy, which creates a discrepancy.
- When communicating with a healthcare professional, it is always specified as “WA” or “WP” to avoid any ambiguity.
Calculating the due date from the date of the last menstrual period
The most common method is to add 280 days to the first day of the last menstrual period. This number corresponds to 40 weeks of amenorrhea. In France, the theoretical due date is often set at 41 WA, or 287 days.
Therefore, 280 is added to 287 days according to the convention used by the practitioner. The distinction between 40 and 41 WA depends on local protocols. The most reliable remains to align with the date communicated after the dating ultrasound.
The concrete steps to follow
Locate the first day of the last menstrual period on a calendar. Count 40 full weeks. You will obtain an estimated due date, which will be confirmed or adjusted during the first ultrasound.
Online calculators automate this operation. You enter the date, and the tool displays the due date, the current trimester, and sometimes the key ultrasound dates.
Irregular cycles and the limits of the calculation by date of last menstrual period
The calculation by the date of the last menstrual period assumes a 28-day cycle with ovulation on the fourteenth day. This assumption is false for a large portion of women. Long cycles, short cycles, late ovulation: dating based on the LMP often overestimates gestational age when cycles are irregular.
Some recent calculators incorporate an adjustment based on the actual cycle length. If you input a 35-day cycle instead of 28, the tool automatically shifts the presumed ovulation and corrects the due date.
The first trimester ultrasound recalibrates everything
The first ultrasound, performed between 11 WA and 13 WA + 6 days, measures the crown-rump length of the embryo. This measurement provides a reliable dating within a few days.
When the gap between the date calculated by the last menstrual period and the ultrasound measurement exceeds a week, the ultrasound dating takes precedence in determining the due date. This point has a direct consequence on decisions regarding induction in case of overdue: recent recommendations emphasize that the decision should be based on ultrasound dating and not solely on the calculation by the LMP.
Screening and ultrasounds: the benchmarks that follow from the calculation
Once the due date is established, the dates for follow-up examinations follow in sequence.
- Combined screening for Down syndrome is recommended between 11 WA and 13 WA + 6 days, at the same time as the first ultrasound.
- The second ultrasound takes place between 22 and 24 WA, known as the morphological ultrasound.
- The third ultrasound is scheduled between 32 and 34 WA to assess the baby’s growth and position.
These windows are calculated based on the established due date, thus indirectly from the date of the last menstrual period, possibly corrected by the dating ultrasound. Missing an examination window because of a two-week miscalculation is a real risk when confusing WA and WP.
The initial calculation by the date of the last menstrual period remains the starting point for all pregnancy follow-up, even if it will almost always be refined. Retaining this date accurately from the start facilitates each step, from the first appointment to the scheduling of the due date. If you have any doubts about the regularity of your cycles, mention it during the first consultation: the first trimester ultrasound will clarify any uncertainty.