Put in your clients' due dates and see the shape of your next few months: where your on-call windows stack up, which weeks are genuinely free, and the odds that two clients labor at the same time. Nothing is saved and nothing is sent anywhere.
Add two or more due dates to see where your windows overlap.
This runs your actual due dates against the real distribution of how far from the estimated date American babies arrive. It is not a guess about your clients; it is what the arithmetic says about dates spaced the way yours are.
Two different things are happening on this page, and they are worth separating, because one is arithmetic and one is probability.
Your on-call window for a client runs from the week you set, counted in her pregnancy, until the week you stop. At a trigger of 36 weeks and an end of 42, that is a window of six weeks per client, positioned by her estimated due date. Draw one bar per client, line the bars up on a calendar, and the overlaps are simply where the bars sit on top of each other. No assumptions are involved. This is the part that tells you how many weeks you are on call for somebody, how deep the stacking gets, and which weeks nobody has a claim on.
Most doulas have never seen this drawn out, and it is usually the part that surprises them. Windows overlap far earlier than the due dates suggest, because you are not comparing single days, you are comparing multi-week spans. That is also the reason there is a ceiling on how many births a month you can take.
The second number is the chance that two of your clients give birth close enough together that you cannot be at both. To work that out you need to know how far from an estimated due date babies actually arrive, and that is published.
The distribution below is every birth registered in the United States in 2023, all 3,596,017 of them, grouped by gestational age at delivery.
| Gestational age at birth | Share of US births, 2023 |
|---|---|
| Under 34 weeks | 2.76% |
| 34 to 36 weeks | 7.64% |
| 37 to 38 weeks | 29.84% |
| 39 to 40 weeks | 54.94% |
| 41 weeks | 4.56% |
| 42 weeks and over | 0.26% |
| All births | 100.00% |
Source: Osterman MJK, Hamilton BE, Martin JA, Driscoll AK, Valenzuela CP. Births: Final Data for 2023. National Vital Statistics Reports, vol 74, no 1. Hyattsville, MD: National Center for Health Statistics; 18 March 2025. Table 20. The 37-to-38 week band is split into single weeks using the singleton figures in NCHS Health E-Stat 104, which reports 12.05% at 37 weeks and 17.58% at 38 weeks for 2023.
The calculator gives each of your clients that distribution, centered on her own due date, and then runs the whole set of them many thousands of times to count how often two births land within 48 hours of each other. The figure you see is the share of those runs that produced a collision.
Whose births these are. This is every registered birth in the country, which includes inductions and scheduled cesareans. That is the right basis for a doula, because you attend those births too and they are part of why the modern distribution sits earlier than it used to. It also means the numbers describe American births in general and not doula clients in particular. If your practice is mostly planned home births, expect your own births to run slightly later than this model does.
It cannot tell you whether you can handle the load. Two clients laboring in the same week is manageable for one doula and impossible for another, and the difference is usually whether there is a reliable backup rather than anything about the calendar. The output here is meant to inform a decision you make, not to make it.
It also has nothing to say about the rest of your practice. A month with no overlapping births can still be a hard month. If what you are really trying to work out is how many clients your income needs rather than how many your calendar allows, the doula rate calculator is the better starting point, and how much should a doula charge works through the reasoning.
There is no fixed number, because it depends on when you go on call. At a 36-week trigger each client occupies about six weeks of your calendar, so two births a month means your windows cover the month roughly twice over. Going on call later shortens each window and reduces how deep the stacking gets.
This calculator shows the answer for your actual due dates rather than in the abstract.
It depends almost entirely on how far apart their due dates are. For two clients due a week apart, the chance they give birth within 48 hours of each other is roughly one in eight. At three weeks apart it falls to about one in twenty-three, and at a month apart to about one in sixty.
Those figures come from the distribution of gestational age at birth for every birth registered in the United States in 2023, and the calculator on this page runs the same arithmetic against your own dates.
Not as close as the phrase 'due date' suggests. In 2023, 29.84% of US births happened at 37 to 38 weeks and 54.94% at 39 to 40 weeks, with 10.41% before 37 weeks and 4.82% at 41 weeks or later. An estimated due date is the middle of a spread of several weeks, not an appointment.
This is why spacing clients neatly across a month does not protect you the way it feels like it should.
It reduces the odds, it does not remove them. Spacing changes how likely a collision is, and the effect is large: moving two clients from one week apart to one month apart cuts the chance of a 48-hour collision by about seven times. But no spacing gets you to zero, because a baby can arrive three weeks early and another three weeks late.
A reliable backup doula is what covers the case spacing cannot.
DoulaFlow tracks gestational age, on-call status and backup cover for every client, so the picture above stays current without you rebuilding it. Fourteen days free, no card.
Start free trial