Calculate last period based on due date

How do I use the pregnancy calculator?

To find out your due date, you just need to select the date of the first day of your last period on our pregnancy calculator tool, adjust your average cycle length and click 'submit’. Your cycle length is the number of days between your periods, counting the first day of your period until the day before your next period starts.

Table of Contents Show

How do I use the pregnancy calculator?
What if I have irregular periods?
How accurate is the due date?
Will I get a more accurate due date later in the pregnancy?
What should I do next?
BabyCenter's Due Date Calculator
How is my due date calculated?
First day of your last period
Conception date
IVF transfer date
Can my due date change?
What if I already know my due date?
How likely am I to give birth on my due date?
How soon can I take a pregnancy test?
How do I calculate my last period?
When did I conceive based on due date?
How many weeks from due date is LMP?

You will then receive your estimated due date and the option to sign-up to our pregnancy email. This weekly email gives you fun pregnancy facts, useful tips and great recipe ideas to help you and your baby have the healthiest possible pregnancy.

Are you in early pregnancy? Find out all you need to know about pregnancy from week 4 onwards.

What if I have irregular periods?

If you were having irregular periods before you got pregnant, you can still use the pregnancy calculator. Just choose your average cycle length. But bear in mind that an ultrasound will be the most accurate way of predicting your due date.

How accurate is the due date?

The pregnancy calculator gives you an approximate due date. It is worked out by adding 280 days (40 weeks) to the first day of your last menstrual period. This is based on a 28-day cycle but is adjusted for longer or shorter cycles.

Your period and ovulation are counted as the first 2 weeks of pregnancy. If you have the baby on your due date, your baby has been growing for 38 weeks, not 40, but you are still classed as 40 weeks pregnant.

Remember that only around 5% of babies are born on their due date so it’s just an estimation.

Will I get a more accurate due date later in the pregnancy?

You will have your first appointment with the midwife, the 'booking appointment', by 10 weeks. At that appointment, you will be offered an ultrasound scan – this is often called the dating scan.

This scan will check how many weeks pregnant you are and work out a more accurate due date. You’ll also be able to find out whether you’re having more than 1 baby and how your baby is developing.

But if you're impatient to know when your baby will arrive, our due date calculator can give you an estimated due date now!

What should I do next?

If you don’t already have a midwife, you may be able to self-refer to your local maternity unit directly for your antenatal care. Visit your local hospital’s website to find out more.

Speak to your GP if you can’t self-refer or if you think your pregnancy may be high-risk. For example, if you have a long-term condition, had complications in a previous pregnancy or experienced a late miscarriage or stillbirth. Your GP will tell the midwifery team you are pregnant.

You will be given a date for your first appointment with a midwife. This should happen before you are 10 weeks pregnant.

We know pregnancy can be an overwhelming time. We created our Healthy Pregnancy tool to try to take away some of this anxiety by giving you a personalised set of tips for your pregnancy.

Use the drop-down menu to calculate your due date or current week of pregnancy.

This calculator allows you to calculate the following:

Last menstrual period (LMP): The LMP calculation uses the standard method of the first day of your last menstrual period plus 280 days to calculate the due date.

Ultrasound due date: If a due date has been calculated from an ultrasound in the first trimester, the reverse calculation method is preferred. This calculates the current gestational age and gives an estimated date of LMP.

Conception date: If conception is known (eg, date of insemination, ovulation or egg retrieval), then the due date can be calculated by adding 266 days.

Gestation at a future or past date: Use to calculate gestation in the future or past. First, calculate current gestation using one of the other methods, then go to this section and choose past or future date.

Date at future or past gestation: Use to calculate the date at a specific gestation (enter in weeks). First, calculate current gestation using one of the other methods, then go to this section and choose a gestational age.

BabyCenter's Due Date Calculator

Use our pregnancy due date calculator by plugging in either the date of your last menstrual cycle or the date you know you conceived. The calculator will do the rest.

How is my due date calculated?

There are several ways your due date is determined. If you happen to know the day you conceived, you can count 38 weeks from that day to find your due date. (Human gestation takes about 38 weeks.)

But very few expectant moms know exactly when they conceived. Even if you only had sex once during your fertile period, you wouldn't conceive on that day unless you happen to be ovulating. Sperm can live for up to five days inside your fallopian tubes. So, it could be up to five days after you have sex that you release an egg (ovulate) and it gets fertilized by a waiting sperm. That's the day you conceive.

So, without knowing the day of conception, how does anyone determine a due date?

First day of your last period

The most common way to calculate your pregnancy due date is by counting 40 weeks from the first day of your last menstrual period (LMP). And that's how most healthcare providers do it.

If your menstrual cycle length is the average length (28-day cycle), your menstrual cycle probably started about two weeks before you conceived. This explains why pregnancies are said to last 40 weeks instead of 38 weeks.

This method doesn't take into account how long your menstrual cycle actually is or when you think you might have conceived. But generally speaking, women typically ovulate about two weeks after their menstrual cycle starts. And women are more likely to know when their last period started than the day they ovulated.

Conception date

If you do happen to know precisely when you conceived – say, if you were using an ovulation predictor kit or tracking your ovulation symptoms – you can calculate your pregnancy due date based on your conception date. Just choose that calculation method from the pulldown above and put in your date.

Note: Again, you don't necessarily conceive on the day you have sex.

IVF transfer date

If you conceived through IVF, you can calculate your due date using your IVF transfer date. If you had a Day 5 embryo transfer, count 261 days from your transfer date. If you had a Day 3 embryo transfer, count 263 days.

Can my due date change?

Your healthcare provider might revise your due date if your baby is measured during a first trimester ultrasound scan and found to be much bigger or smaller than expected for gestational age. This is more likely to happen if you have an irregular menstrual cycle length that makes it hard to pinpoint the date of conception.

Your healthcare provider will measure your baby during that ultrasound exam to figure out how far along your baby is and then provide you with a new due date.

What if I already know my due date?

If you already know your due date, you can use this calculator to see your pregnancy timeline. It will tell you when you'll hit various milestones, and when you may be due for prenatal tests and prenatal visits. You'll also find what your baby's sign and birthstone will probably be and which famous people were born on your due date.

How likely am I to give birth on my due date?

Of course, a due date calculation is always approximate, whether it's from our tool or from your doctor or midwife. Only 1 in 20 women delivers on their due date. You're just as likely to go into labor any day during the two weeks before or after.

Want more information about how the weeks, months, and trimesters of pregnancy are counted? See our pregnancy timing chart.

How soon can I take a pregnancy test?

With all this talk about pregnancy due dates, you may be wondering when you can take a pregnancy test. To ensure you get the most accurate reading, it's best to wait a few days after your missed period to take a pregnancy test.

At-home urine tests measure the amount of hCG (human Chorionic Gonadotropin) present in your body. If you take a pregnancy test before you miss your period, you may not get an accurate result, despite what some tests advertise.

If you're getting a blood test in your provider's office, you may get results sooner. These tests also measure the amount of hCG in your bloodstream, but they're more sensitive than at-home urine tests. Blood tests may be able to detect pregnancy six to eight days after ovulation.

Read more

Your pregnancy, week by week
Your first trimester pregnancy checklist
Pregnancy Weight Gain Calculator
Ovulation Calculator
See all tools

How do I calculate my last period?

Measure how long your average cycle is, i.e., the time between one period beginning and the next one beginning. If you are unsure, use the average value of 28 days. Note how long your average period duration is. If you do not know this value, use the average of 6 days.

When did I conceive based on due date?

For a regular 28-day cycle, pregnancy is estimated to be 280 days, or 40 weeks, from your last menstrual period, and 266 days, or 38 weeks, from the date of ovulation. Therefore, to calculate the most likely date of conception, you need to subtract 266 days from your estimated due date.