The presumed date of delivery (PDD) is based on a calculation in weeks of amenorrhea (WA) from the date of the last menstrual period (LMP), conventionally set at 40 WA or 280 days. In Belgium, this reference conditions both obstetrical follow-up and the opening of social rights, making the accuracy of the calculation crucial from the first trimester.
Difference between weeks of amenorrhea and weeks of pregnancy: impact on Belgian follow-up
Weeks of amenorrhea are counted from the first day of the last menstrual period. Weeks of pregnancy (WP) start at fertilization, approximately two weeks later. The confusion between these two references regularly causes discrepancies in the interpretation of laboratory results and fetal growth curves.
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In consultations, Belgian gynecologists and midwives almost exclusively use WA. The medical file, the ONE pregnancy booklet, and examination prescriptions all refer to this count. A patient who states “being at 12 weeks” without specifying the reference introduces a two-week ambiguity regarding the actual gestational age.
We recommend always converting mentally: WP = WA minus 2 weeks. This is the only operation to remember for navigating between information found online (often expressed in WP) and the Belgian medical language (always in WA). To delve deeper into the calculation of pregnancy in Belgium, the step-by-step method starting from the LMP remains the most reliable starting point before any ultrasound confirmation.
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Dating ultrasound: why it takes precedence over the LMP
Online calculators apply a simple formula: LMP + 280 days. This approach assumes a regular reference cycle and predictable ovulation. In practice, cycles vary, sometimes by several days, which can shift the PDD by one to two weeks.
The first trimester ultrasound corrects this bias. By measuring the crown-rump length (CRL) of the fetus, the practitioner establishes a more accurate gestational age. When the gap between the ultrasound dating and the dating by LMP exceeds a clinically significant threshold, the ultrasound date becomes the sole reference for all follow-up.
This technical point has concrete consequences in Belgium:
- The official term recorded in the obstetrical file is the one validated by the ultrasound, not the one calculated by an application.
- The corrected PDD serves as the basis for planning morphological ultrasounds in the second and third trimesters.
- The medical certificate sent to the mutual insurance for maternity leave mentions this corrected PDD, which directly impacts the leave dates.
We observe that many patients rely on the date calculated online and are surprised when the ultrasound technician modifies it. The LMP provides an estimate, the ultrasound provides the medical reference date.
PDD and social rights of self-employed women in Belgium
For employees, the mechanics of maternity leave are relatively framed by the employer and the mutual insurance. For the self-employed, the PDD plays an even more structuring role, as it solely determines the schedule for leave.
Mandatory prenatal and postnatal leave
The medical certificate mentioning the presumed date of delivery must be attached to the maternity leave request to the mutual insurance. The mandatory prenatal leave week is calculated directly from this PDD: it covers the seven days preceding it. After delivery, a mandatory postnatal leave of two weeks applies.
Optional weeks and flexibility
Beyond these mandatory periods, optional weeks can be taken. The window for taking them extends over a period that can go up to 36 weeks after the end of the mandatory postnatal leave. A poorly calculated PDD, not corrected by ultrasound, can shift this entire window and complicate the planning of the return to work.
The accuracy of the PDD is therefore not a minor obstetrical detail: it conditions access to optional leave weeks for the self-employed.
Reliability of online tools and limits to be aware of
Online pregnancy calculators remain useful as a first estimate. Their operation is based on the formula LMP + 280 days, sometimes with a variant of 285 days depending on the method used. This divergence of a few days between platforms does not pose a medical issue, but it fuels confusion.
Three technical limits deserve attention:
- No online calculator takes into account the individual variability of the cycle, nor a possible ovulation shift related to a recent cessation of hormonal contraception.
- The WA/WP conversion is not always specified, and some tools display results in WP without clearly mentioning it.
- The results never replace ultrasound dating, which remains the only medical reference act for determining the term in Belgium.
A calculator provides a range. The ultrasound gives a date. The medical certificate formalizes this date for social rights. These three levels of precision are not interchangeable.
For pregnancies obtained through assisted reproductive technology, the date of embryo transfer replaces the LMP in the initial calculation, making public calculators even less relevant. The practitioner who performed the transfer directly provides the starting gestational age.
The calculation of pregnancy in Belgium is situated within a framework where ultrasound dating and the administrative obligations of the mutual insurance intersect. Mastering the difference between WA and WP, having the PDD corrected from the first trimester, and transmitting the correct date on the medical certificate: these three reflexes are sufficient to secure both medical follow-up and access to rights.