Conception Date Calculator NZ 2026
This calculator estimates when conception likely occurred, working either backwards from a due date or forwards from the first day of your last menstrual period, two of the most common starting points. Many people want to know their conception date out of curiosity, to understand the timeline of a pregnancy, or to relate it to events, travel or dates that matter to them. The estimate relies on the standard way pregnancy is dated. Conventionally, pregnancy is counted from the first day of the last menstrual period, and a full-term pregnancy is about 280 days, or 40 weeks, from that day. Conception, however, happens at ovulation, which is typically around 14 days after the last period begins, so conception is usually about 266 days before the due date. Because ovulation timing varies from person to person and cycle to cycle, conception is best thought of as a window of a few days rather than a single exact day. You choose whether you are working from your due date or your last period, enter the date, and the calculator estimates your conception date, a conception window of roughly five days around it, and the related date, either the estimated due date or the last period, to complete the picture. Use it as an interesting and useful guide, bearing in mind it is an estimate built on average cycle assumptions. Dating scans, especially an early ultrasound, give a more accurate picture and are what your midwife or doctor will rely on, particularly if your cycles are irregular or you are unsure of your dates. This tool is for general information and is not a medical dating of your pregnancy.
An estimate based on average cycle timing. Conception is a window, not an exact day. An early dating scan is more accurate. Not medical advice.
How it works
Pregnancy is dated from the first day of the last period, with a due date about 280 days later and conception about 266 days before the due date, since ovulation is roughly 14 days after the period begins. Working from a due date, conception is the due date minus 266 days. Working from the last period, conception is that date plus about 14 days and the due date is 280 days later.
Worked example
With a due date of 1 December, conception is estimated about 266 days earlier, around 9 March, with a likely window spanning a few days either side. The estimated last period would be about 24 February. Working instead from a last period of 24 February, conception would be estimated around 10 March and the due date around 1 December.
Related calculators
- Due Date Calculator: estimate your due date.
- Ovulation Calculator: fertile window.
- Pregnancy Weight Gain: healthy gain range.
- Age Calculator: work out an age or span.