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Ovulation, Your Cycle and Due Dates: A Practical Guide

Last reviewed: 25 June 2026 · ~7 min read

Whether you're trying to conceive, avoid it, or simply understand your body better, knowing how your cycle works turns a confusing monthly mystery into something predictable. This guide walks through the phases of the menstrual cycle, pinpoints when ovulation actually happens, explains the fertile window, and shows how pregnancy due dates are estimated — in plain language, with the calculators that do the date maths for you.

The menstrual cycle in four phases

A cycle is counted from the first day of one period to the first day of the next. The textbook length is 28 days, but anywhere from about 21 to 35 days is common and normal. Each cycle moves through four phases:

The key insight: the luteal phase is fairly fixed at about 14 days, but the follicular phase varies. That's why ovulation is best estimated by counting back ~14 days from your next expected period — not by assuming it lands mid-cycle.

When does ovulation happen?

Ovulation usually occurs about 14 days before your next period. In a 28-day cycle that's roughly day 14; in a 32-day cycle it's closer to day 18. Our ovulation calculator does this for you: enter the first day of your last period and your average cycle length, and it estimates your ovulation day and fertile window. Signs that ovulation is near can include a small rise in basal body temperature after it happens, and clearer, stretchier cervical mucus in the days before.

The fertile window

You can only conceive around ovulation, but the window is wider than a single day because of biology: sperm can survive in the body for up to about five days, while the egg lives for only 12–24 hours after release. Put together, that gives a fertile window of roughly six days — the five days leading up to ovulation plus ovulation day itself. The days just before ovulation tend to be the most fertile.

If you're trying to conceive, timing intimacy across that window — rather than trying to hit one perfect day — is the practical approach. If you're trying to avoid pregnancy, be aware that cycle-tracking alone is far less reliable than dedicated contraception, because cycles shift and ovulation can move.

From conception to due date

Once pregnancy is confirmed, the next question is usually "when is the baby due?" The standard estimate, known as Naegele's rule, adds 280 days (40 weeks) to the first day of your last menstrual period. Our due date calculator applies this instantly. Because the rule assumes a textbook 28-day cycle with ovulation on day 14, it's only an estimate — an early ultrasound, which measures the embryo directly, typically gives a more accurate date. Only about 1 in 20 babies actually arrives on the estimated due date; most come within a week or two either side.

Through the pregnancy

Healthy weight gain supports a healthy pregnancy, and the right amount depends on your starting weight. The pregnancy weight gain calculator gives a recommended range based on your pre-pregnancy BMI, following mainstream clinical guidance. As always, your own care team's advice takes priority over any general guideline.

Frequently asked questions

When do you ovulate?

About 14 days before your next period — not necessarily mid-cycle. In a 28-day cycle that's around day 14, but it shifts with cycle length.

How long is the fertile window?

About six days: the five days before ovulation plus ovulation day, because sperm survive several days while the egg lives only 12–24 hours.

How is a due date calculated?

The common estimate adds 280 days (40 weeks) to the first day of your last period. It assumes a 28-day cycle, so an early ultrasound is usually more accurate.

This article is general education, not medical advice, and cannot replace care from your doctor or midwife. For contraception, conception difficulties, or any pregnancy concern, consult a qualified professional. See our Terms & Disclaimer.

Calculators used in this guide