Free Pregnancy Tools

Pregnancy & Fertility Calculators, Explained

Belly Bell is a free collection of pregnancy and fertility tools built to answer the questions that come up the moment you start trying to conceive or discover you're expecting: When is my baby due? Which days am I most fertile? How many weeks along am I? Every tool runs entirely in your browser — no account, no data stored on a server, and no cost. Below is a short guide to how each calculation works and how to read its results.

How the Due Date Calculator Works

The due date calculator uses Naegele's rule, the same method clinicians use: it counts 280 days (40 weeks) from the first day of your last menstrual period (LMP). Because this assumes a regular 28-day cycle with ovulation on day 14, your real due date can shift if your cycles are longer or shorter. Only about 1 in 20 babies actually arrives on the calculated date — most arrive within the two weeks on either side, which is why doctors treat it as an estimated window, not a fixed appointment.

Ovulation & Fertile Window

The ovulation and fertile window tools estimate the days you're most likely to conceive. Ovulation typically happens about 14 days before your next period starts. Because sperm can survive up to five days and an egg lives roughly 12–24 hours, your fertile window spans the five days before ovulation plus ovulation day itself. Tracking it across a few cycles gives a far more reliable pattern than any single estimate, especially if your cycle length varies.

Tracking Your Pregnancy Week by Week

The pregnancy week calculator and conception calculator work backwards and forwards from your LMP or known conception date to show how far along you are and which trimester you're in. Pregnancy is measured in gestational weeks counted from your LMP — which is why you're considered "two weeks pregnant" before conception even occurs. Our baby gender predictor is based on the traditional Chinese gender calendar and is offered purely for fun — it has no scientific basis and should never replace an ultrasound or medical test.

A Note on Accuracy

These calculators provide estimates for educational purposes only. They are not a substitute for prenatal care, ultrasound dating, or advice from a qualified midwife or obstetrician. If you are pregnant or trying to conceive, your healthcare provider can give you a far more precise due date and personalised guidance. If anything about your pregnancy concerns you, contact your provider directly rather than relying on an online estimate.

Frequently Asked Questions