Reverse Due Date Calculator

Q: How does the reverse due date calculator work?

The reverse due date calculator takes your birthday (or any birth date) and works backwards: your estimated due date is your birthday (since you were born on your due date or close to it); conception is approximately 266 days before your birthday; and the last menstrual period was approximately 280 days before your birthday.

Q: What does my birth season tell me about my conception?

Birth in winter (December–February) suggests conception in spring (March–May). Summer births (June–August) suggest autumn conception (September–November). This is approximate and based on a standard 40-week pregnancy.

Q: What is the most common birth week?

Spontaneous births (without induction or caesarean) most commonly occur at 39–40 weeks gestation. The ONS data shows the single most common week for UK births is week 39.

Q: What is a post-term pregnancy?

A post-term pregnancy is one that continues beyond 42 weeks. In the UK, the NHS offers induction of labour at around 41–42 weeks to reduce risks. Only about 2–7% of pregnancies go beyond 42 weeks.

Enter any birthday to work backwards and find the estimated due date, conception date, and last menstrual period — a fun way to explore the origins of any birth date.

Enter a Birthday

Birthday (or any birth date)

You can enter your own birthday, a child's birthday, or any historical date.

Pregnancy Details for This Birthday

Fun Facts About This Birthday

Important: These are estimates based on a standard 40-week pregnancy. Actual birth can occur anywhere from 37–42 weeks, so real dates may differ by up to 3–4 weeks.

How the Reverse Due Date Calculator Works

A reverse due date calculator does the opposite of a standard pregnancy calculator. Instead of starting with a known last period date and calculating the due date, it starts with a known birthday and works backwards through pregnancy.

The calculations are based on standard obstetric formulas:

Estimated due date: Your birthday (you were born on or near your due date)
Conception date: Birthday minus 266 days (38 weeks)
Last menstrual period: Birthday minus 280 days (40 weeks)
Conception month: Approximately 9 months before your birth month

Example: Someone born on 25 December 1990 was likely conceived around 31 March 1990 (Easter season), and their mother's last period was approximately 17 March 1990.

Why the Calculation Is Approximate

A reverse due date calculation is an interesting estimate, but it comes with significant caveats that are worth understanding:

The 37–42 Week Range

A "full term" pregnancy is defined as any birth between 37 and 42 weeks gestation. This 5-week window means that even if you know the exact due date, the birth could occur weeks before or after. Conversely, knowing a birth date tells you the birth happened — but not at which gestation.

Gestational Age at Birth	Classification	UK Incidence
Before 28 weeks	Extremely premature	~0.5%
28–31 weeks	Very premature	~0.7%
32–36 weeks	Moderately premature	~6–7%
37–38 weeks	Early term	~20%
39–40 weeks	Full term (most common)	~50%
41 weeks	Late term	~15%
42+ weeks	Post-term	~2–3%

Induction and Caesarean Sections

In the UK, approximately 33% of births are by caesarean section (planned or emergency) and around 30% of labours are induced. Planned caesareans and inductions are often scheduled before 40 weeks, which means many births occur before the estimated due date by design, not by premature labour.

Season of Birth and Conception

Working backwards from your birth month gives you a rough idea of when you were conceived. Here is a seasonal guide:

Birth Season	Birth Months	Likely Conception
Winter	December, January, February	March, April, May
Spring	March, April, May	June, July, August
Summer	June, July, August	September, October, November
Autumn	September, October, November	December, January, February

Research shows that birth months are not evenly distributed across the year. In the UK, September is statistically one of the most common birth months, suggesting peak conception occurs around December — which may reflect longer nights and festive celebrations.

Season of Birth Effects

Researchers have identified associations between birth season and various outcomes, though these are statistical patterns, not individual predictions:

School readiness: Children born in August (just before the September school year cutoff) are the youngest in their year group and on average underperform compared to September-born classmates — a well-documented "relative age effect".
Vitamin D: Babies born in autumn and winter may have lower vitamin D stores due to reduced sun exposure for their mothers during the third trimester.
Multiple sclerosis: Some research links spring birth (specifically May in the Northern Hemisphere) with slightly higher MS risk — possibly related to winter pregnancy and vitamin D deficiency.
Sporting achievement: Athletes disproportionately born in the first quarter of their school year have better early development opportunities (the relative age effect).

Birth season associations are population-level statistics. They do not determine any individual's outcomes or abilities.

UK Birth Statistics

The Office for National Statistics (ONS) publishes detailed annual birth data for England and Wales. Some interesting patterns from recent data:

Approximately 600,000–700,000 babies are born in England and Wales each year
September is consistently one of the most popular birth months
Tuesday is the most common birth day (reflecting weekday inductions and elective caesareans)
Most spontaneous labours begin at night or early morning
The average gestation at birth is approximately 39 weeks and 5 days in the UK
First-time mothers tend to give birth on average 8 days after their due date if labour begins spontaneously

Zodiac Sign from Birthday

If you are curious about the zodiac sign associated with a birth date, here is the UK/Western astrology breakdown:

Sign	Dates
Aries	21 March – 19 April
Taurus	20 April – 20 May
Gemini	21 May – 20 June
Cancer	21 June – 22 July
Leo	23 July – 22 August
Virgo	23 August – 22 September
Libra	23 September – 22 October
Scorpio	23 October – 21 November
Sagittarius	22 November – 21 December
Capricorn	22 December – 19 January
Aquarius	20 January – 18 February
Pisces	19 February – 20 March

Frequently Asked Questions

How does the reverse due date calculator work?

The reverse due date calculator takes a birth date and works backwards through pregnancy. Your birthday is treated as your approximate due date. Conception is estimated at 266 days before your birthday (38 weeks from conception to birth), and the last menstrual period at 280 days before (40 weeks from LMP to birth). These are averages — actual dates vary.

How accurate is a reverse due date calculation?

It is an approximation only. Babies are born anywhere from 37 to 42 weeks gestation, meaning the estimate may be off by 2–5 weeks. Inductions and planned caesareans also affect birth timing. The calculation is best treated as a fun estimate rather than a definitive date.

What does my birth season tell me about my conception?

Working backwards approximately 9 months gives a rough conception season. Winter births (December–February) suggest spring conception (March–May); summer births (June–August) suggest autumn conception (September–November). This is based on the 40-week average pregnancy length.

What is a premature birth?

A premature birth is any birth before 37 weeks of gestation. Extremely premature: before 28 weeks. Very premature: 28–31 weeks. Moderately premature: 32–36 weeks. In the UK, approximately 7–8% of babies are born prematurely. Premature birth means the reverse calculation would be even less accurate, as the baby was born earlier than their due date.

What is the most common birth week?

Spontaneous births most commonly occur at 39–40 weeks gestation. The single most common week for spontaneous labour in UK data is week 39. However, when induced and caesarean births are included, 38–39 weeks becomes more common as many planned births are scheduled just before the due date.

What is a post-term pregnancy?

A post-term pregnancy continues beyond 42 completed weeks. In the UK, the NHS offers induction of labour at around 41–42 weeks to reduce risks (including an increased risk of stillbirth after 42 weeks). Only about 2–7% of pregnancies naturally continue beyond 42 weeks today, partly because induction is routinely offered.

Can I use this for historical birthdays?

Yes — the calculator works for any birth date, including historical dates. It is a fun way to estimate when historical figures may have been conceived. Just remember that historical calendars, record-keeping, and birth practices differed significantly, so results are especially approximate for older dates.

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Last updated: February 2026. This calculator is for fun and general interest. Dates are estimates based on standard 40-week pregnancy calculations. Written by Mustafa Bilgic.