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Reverse Due Date Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Lawson G. W. Naegele's rule and the length of pregnancy - A review.; The Australian & New Zealand journal of obstetrics & gynaecology; October 2020See 1 more source
Wilcox A.J.; Dunson D.; Baird D.D. The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study.; BMJ; November 2000
Last updated: Jan 18, 2024

The reverse due date calculator will help you no matter if you're already pregnant, planning to conceive, or just curious to find out the day you were created, to discover how old you truly are. ⭐

We'll help you with:

  • Calculating your conception date based on your birthday;
  • Planning the conception date, so you can obtain the due date of your desire.

Read on to discover the full range of our reverse conception calculator's functions (instructions included! 📝).

How to use the reverse due date calculator?

Our reverse due date calculator only requires one value from you. It can be either:

  • Your birthday date;
  • Your due date;
  • Your conception date; or
  • The date intercourse occurred.

Omni Calculators work both ways – you can enter your data anywhere you want!

💡 Check the advanced mode button if the pregnancy has already happened, and you know exactly how long it lasted.

Our reverse pregnancy due date calculator will show you an estimated time interval, where the intercourse most likely occurred/should occur.

Why is it an interval and not a single date?

Human fertility is a complicated subject, and it depends on many unpredictable variables. That's why we always have to give ourselves some uncertainty range to avoid any possible errors.

So what may go wrong?

  • Ovulation does not always happen on the very same day each month; and
  • Conception does not have to occur on the calculated day of ovulation.

How to calculate the conception date based on birthday?

Firstly, we need to make a few assumptions:

  1. Menstrual cycle length is equal to 28 days; and
  2. Human pregnancy lasts exactly 40 weeks, that is 280 days.

(These are the common assumptions we need to make in obstetrics - for example, in Naegele's rule for assessing the gestational age and the estimated delivery date.)

The equation we need looks as follows:

Conception = Birthday date - 280 days


Conception = Due date - 280 days

Now we need to estimate the error range for the intercourse date; let's calculate the earliest possible day the intercourse occurred:

Intercourse = Conception - 7 days

💡 If you'd like to be more precise with your calculations, we'd recommend you find out on exactly what week of the pregnancy you were born. The estimate presented above may be especially misleading for babies born preterm! - check the advanced mode.

Planning your pregnancy calendar

Is planning your pregnancy possible? Of course, it is (to a reasonable level)! We'll guide you through the process of using all the resources available to help you obtain the best results possible.

  1. Intercourse date

    Do you already know your ovulation dates? If not, our ovulation calculator might come in handy. For confirmation that the ovulation did happen, you may use a urine ovulation test or a temperature and cervical mucus methods. You could also use our conception due date calculator in reverse by choosing your desired birthday.

    For intercourse to produce a baby, it must happen up to 24 hrs after ovulation.

💡 Contraceptives are an efficient way to choose the perfect time for a pregnancy - with oral contraceptives, you can get pregnant the month after you stop using them.

  1. Implantation: + 7 days

    The embryo uses this time for the long journey between the fallopian tubes and the uterus. A few days after this, your pregnancy can be detected with a simple, store-bought urine hCG level test.

  2. First ultrasound: + 11/13 weeks

Now, as the baby is rapidly developing, we must check that everything is alright. Your doctor will measure the CRL (length) of the embryo. You will also be able to hear the baby's heartbeat just after the 6th week of pregnancy! 💓

  1. Second ultrasound: + 18/22 weeks

The doctor will check on the baby's abdominal and head circumference and femur length and will rule out all possible abnormalities. You may also want to check if your pregnancy weight gain is correct!

  1. Third ultrasound: 28/32 weeks

This is the final assessment of the child's well-being, as well as its pace of development. If you haven't done it before, this is the final time you can get all of the tests required for pregnant women – the presence of viral diseases, bacterial diseases, and diabetes in pregnancy.

  1. Delivery: + 280 days

Use our due date calculator to obtain your predicted due date. We know that not every child will be conceived naturally – which is why we made a special IVF delivery date calculator.

Are you all packed and prepared? Delivery is a long journey. We're all keeping our fingers crossed for you!


How do I calculate the reverse due date?

To calculate the reverse due date, you can follow the next steps:

  1. Take the estimated date of birth or due date.
  2. Subtract 280 days from the date determined in Step 1 to calculate the conception date.
  3. Subtract 7 days from conception to determine the earliest possible day the intercourse occurred.
  4. That's all! You have now calculated the reverse due date.

What is my conception date if my due date is August 1st, 2023?

If your due date is August 1st, 2023, your conception date is estimated to be October 25th, 2022. If you're interested in calculating the earliest possible day the intercourse occurred, you can subtract seven days from October 25th, which equals October 18th, 2022.

When should I conceive if I want a baby in July 2024?

If you want your baby to be born in July 2024, the earliest possible date of intercourse should be between September 18th, 2023, and October 18th, 2023. These results are based on the assumption that your menstrual cycle length equals 28 days and your pregnancy will last 40 weeks.

Can due date be wrong by 2 weeks?

Yes, your due date can be wrong by two weeks; the reasons for a due date change can vary because of various factors, including irregular periods, abnormal fundal height measurements, or atypical levels of alpha-fetoprotein (a protein produced by the baby).

Łucja Zaborowska, MD, PhD candidate
Due date
Conception date
Intercourse date
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