The Math of Fertility and Pregnancy

Fifty calculators, one question

In October 2026 I tested 50 of the most widely used online due date calculators. Each one got the same four test pregnancies, entered in the same way, on the same day. The question was simple: does it give the right date?

Ten of the 50 blocked automated testing and two could not be tested for other reasons. Of the 38 that could be tested, six gave a wrong due date or a wrong week of pregnancy, and four did not work at all. Most errors were one day. One day sounds small. It is not. ACOG defines late term as starting at 41 weeks 0 days and post-term at 42 weeks 0 days, and recommends delivery after 42 weeks 0 days and by 42 weeks 6 days. 23 A due date that is off by a day moves every one of those lines.

These are not hard calculations. That is exactly the point. When the easy math goes wrong, nobody checks it, because everybody assumes it is right.

What I mean by the math of pregnancy

Obstetrics looks like a field of hands, monitors and judgment. Underneath, it runs on arithmetic. Before any clinical decision there is a number, and almost every number belongs to one of five kinds.

Time: how far along is this pregnancy today, and when is the baby due? Chance: what is the probability of conceiving this month, or that this hCG level is normal? Size: is this baby growing as expected? Risk: what does a 2-fold increase mean for this woman? Scores: what do the signs add up to, at the bedside, in seconds?

Fertility is the same. A cycle is a calendar. Ovulation is a date. A semen analysis is a set of counts compared against reference limits. Ovarian reserve is a lab value read against age. The physiology is biology. The decisions are math.

The calculators

Counting time 6

Counting chance 7

Growth and monitoring 6

Weighing risk 10

Labor and birth 7

Fertility, cycle and hormones 7

Mental health scores 4

How I build the math

Nearly 120 free tools live at tools.obmd.com, 47 of them calculators, timers and scores, and every one follows the same rules.

1. Every formula traces to a publication. If a number cannot be traced to a specific source, it does not appear.
2. The source is on the page. You should never have to trust a number you cannot check.
3. No extrapolation. When the published data stop, the tool stops. A blank is more honest than a guess.
4. Absolute numbers first. Every risk comes with its denominator.
5. Tested with fixed cases. Each calculator is checked against answers computed independently, including the days around the clock changes, before it goes live.
6. Calculated on your device. What you type stays in your browser.