The labor calculator reports probabilities conditional on today's date. Each time it runs, the days that have already passed without labor are removed from the distribution and the remaining probability is rescaled to 100%. The result describes the forecast from today forward, and it changes as today changes.
The due date does not move. The forecast around it does.
Conditional probability
The underlying distribution assigns a probability to every day from 20 weeks through 43 weeks 2 days. Seen from early pregnancy, the 40th week holds about 28% of the probability in the baseline model for a first pregnancy, and the days before 37 weeks hold 5%.
Once a pregnancy reaches 37 weeks, the 5% assigned to earlier days can no longer occur. The remaining 95% is rescaled so that it sums to 100%, and every later day's probability rises by about 5% of its former value. At 40 weeks, more than half of the original distribution has been passed, and the remaining days are rescaled by a larger factor. Smith (2001) used the same time-to-event approach to estimate the normal length of pregnancy, and the calculator's conditional figures follow the same logic.
Three visible effects
The window moves later. Its start cannot be earlier than today, and as early days are removed, the shortest span holding half of the remaining probability shifts. In the baseline model, the 50% window calculated at 36 weeks runs from about 39 weeks 5 days to 41 weeks 3 days. Calculated at 40 weeks 0 days, it runs from about 40 weeks 1 day to 41 weeks 2 days.
The window narrows. Half of a shorter remaining distribution fits within fewer days. The baseline window is about 12 days wide at 36 weeks and about 8 days wide at 40 weeks.
The 10% date approaches. At 36 weeks, there is a one-in-ten chance that labor begins by 38 weeks 6 days. At 39 weeks, the 10% date is 39 weeks 4 days. At 40 weeks it is 40 weeks 2 days, two days away. The near-term figures on the result page, the chances of labor within one, three, seven or fourteen days, rise in the same way.
What does not change
The due date is a dating convention and is not affected by the forecast. The answers entered on the form do not change unless the user changes them. The underlying distribution does not change. Only the portion of it that remains relevant does.
The median of the remaining forecast moves later slowly. At 36 weeks it is 40 weeks 5 days in the baseline model. At 40 weeks 0 days it is 41 weeks 1 day. A pregnancy that continues past its median does not become overdue in the sense of the forecast failing. The model expected half of pregnancies to pass that point.
Late pregnancy
The rescaling has its largest effect after 41 weeks. Marquette and colleagues (2014) reported that 67.6% of uncomplicated pregnancies still ongoing at 41 weeks 0 days began labor spontaneously within the following week. The baseline model gives 64% for a first pregnancy and 68% for a later one at the same point. Beyond 42 weeks the calculator stops accepting calculations, because scheduled delivery is common at that stage and the model does not represent it.
Rerunning the calculator
Because the forecast is conditional on today, the result from a week ago is not the current forecast. Rerunning with the same answers gives the updated one. The calculator can save entries in the browser for 30 days for this purpose, and the saved result is recalculated for the current date each time it is opened. The methodology page describes the distribution and the rescaling in full.