
Due Date Calculator
Excited to know when your little one will arrive? Use our Due Date Calculator to find out this important piece of information.
How to Calculate your due date
Written by Mindsmaking Medical Writer
Fact Checked by Mindsmaking Professionals
24th February, 2025
A typical pregnancy lasts around 40 weeks, but the duration can vary. This variation can make due date estimation a bit tough. Therefore, it's typical to utilize a due date calculator to estimate the time of your baby’s arrival. Due date calculators generally work by adding approximately 280 days to the first day of the last menstrual period (LMP).
Knowing your due date is important because it provides a timeline for monitoring your baby’s growth and development, allows you to plan effectively for prenatal care, labour, and delivery, and helps your healthcare team assess your baby's gestational age to ensure consistent care.
While our due date calculator offers an estimate, it is important to note that it provides a range rather than a definitive date. Just as the pregnancy timeline is subject to natural variations, your baby’s arrival is also subject to the same.
Key Facts
Our due date calculator determines the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP), a common initial estimation method.
An ultrasound scan is the most accurate method for estimating your due date, especially if you are uncertain about your LMP or conception date.
Due dates are adjusted based on early ultrasounds (7–13 weeks) when they differ significantly from the last menstrual period.
A study showed that over half (53.74%) of mothers felt stress about their baby's arrival, indicating the emotional uncertainty surrounding estimated due dates.
How Our Due Date Calculator Works
Estimating your due date is often the first thing you want to know after finding out you're pregnant, and it’s more than just picking a date 9 months ahead. Our calculator combines traditional methods with personalized insights from your cycle and ovulation data for a timeline tailored to your body.
First day of the last period
Most due date calculators (including ours) begin with a method called Naegele’s Rule. This rule works by adding 280 days (or 40 weeks) to the first day of your last menstrual period (LMP) [3]. That’s because, on average, pregnancy lasts about 40 weeks from the start of your last cycle, not from the day you conceived.
Let’s say your last period started on January 1st. Using Naegele’s Rule, your estimated due date would be October 8th.
But here’s the catch: Naegele’s Rule assumes that everyone has a 28-day menstrual cycle and ovulates exactly on day 14. In reality, everybody is different. Some people ovulate earlier or later. Some have 24-day cycles, others have 35-day cycles. That’s where a personalized approach makes all the difference.
Ovulation or Conception Date
The most accurate way to estimate your due date is by knowing when you actually ovulated. That’s because conception usually happens within 12–24 hours after ovulation. So if you ovulated on day 21 of a 35-day cycle, your due date will be a full week later than what Naegele’s Rule suggests.
With our calculator, if you know your ovulation date or conception date, you can input it directly, which gives you a more precise estimate.
Ultrasound Dating
If you've had a first-trimester ultrasound, our due date calculator can estimate your due date based on the scan [6]. You simply enter the date of the ultrasound and the gestational age shown in your report, for example, “9 weeks and 2 days.”
Early first-trimester ultrasound is the most accurate way to determine gestational age and the estimated due date, especially when the last menstrual period (LMP) is uncertain or inconsistent. The American College of Obstetricians and Gynecologists (ACOG) recommends using ultrasound-derived dates over LMP in cases of significant difference between the two dates [2].
The calculator counts forward from that point to determine your estimated due date, which is often more accurate than using your last menstrual period, especially if your cycle is irregular or unknown.
If you're already using our pregnancy tracking app, you can log your ultrasound details directly, and your due date will update automatically for a more personalized experience.
How Fertility Tracking Helps You Get a Personalized Estimate
Tools like basal body temperature charts, ovulation predictor kits (OPKs), and cervical mucus tracking help pinpoint the exact day you ovulate.
If you're already using our cycle and ovulation tracking app, this data flows directly into your pregnancy tracking, giving you a personalized due date from the start.
And if you conceived through fertility treatments like IVF, our calculator lets you enter specific details, such as the embryo transfer date or fertilization date, which allows for extremely accurate results.
While IVF provides a precise conception timeline, studies show that ultrasound-based dating (GAUS) may show the pregnancy as slightly further along. This is likely due to the controlled timing of ovulation and fertilization in IVF, not faster embryo development. Therefore, aligning IVF pregnancies with natural timelines using ultrasound remains a widely accepted and clinically appropriate approach [4].
Why Technology Is Making Due Date Estimation Smarter
Today’s tools go beyond pen-and-paper calculations. Our calculator also supports:
- Irregular periods or custom cycle lengths
- Conception date (if known)
- Fertility treatment dates (like IVF)
- Luteal phase length
- First-trimester ultrasound results (the most accurate medical method)
And emerging tech is taking this even further, recent studies show wearable tech, like smart rings, can help predict ovulation and estimated due dates [8]. That’s why our system blends science, cycle awareness, and your input for a result that’s tailored, not generic.
Get a Due Date That Matches Your Body, Not Just the Average
Whether your cycle is regular or unpredictable, our free due date calculator adapts to your reality. Even better, our fertility and pregnancy tracking app automatically personalizes your pregnancy week-by-week journey from conception through birth.
How do I Calculate My Due Date?
You can calculate your due date based on the first day of your last period, the date of conception, your IVF transfer date, or the timing of your first ultrasound scan.
First Day of Last Period
To calculate your EDD, add 280 days to the first day of the last menstrual period (LMP). For example, if your last period started on January 1st, your estimated due date would be around October 8th. However, this calculation is only accurate if you have a regular 28-day menstrual cycle [6]
Conception Date
If you know the exact date you conceived (which is less common unless you were tracking ovulation, the calculation is slightly different. Instead of adding 280 days from your last period, you add 266 days (or 38 weeks) to your conception date.
For instance, if you conceived on 15th February, your estimated due date would be around 8th November. This method is considered more accurate if the date of conception is known with certainty, but for most people, the LMP method remains the default.
IVF Transfer Date
In the case of a pregnancy achieved through assisted reproductive technologies like In Vitro Fertilization (IVF), it is easier to calculate the due date from the onset. Because you know the dates of egg retrieval and subsequent embryo transfer, you can calculate your EDD by adding the gestational age of the embryo at transfer to the expected full gestation period (typically 266 days from egg retrieval for a day-5 transfer).
This method is highly reliable because the timing of conception is precisely known. For example, if a 5-day embryo were transferred on September 10, 2024, you would add 261 days to that date [5]. However, your healthcare provider may still use early ultrasound measurements to help align your pregnancy timeline with those of natural pregnancies.
This gives an estimated due date of May 29, 2025. If a 3-day embryo was transferred on November 5, 2024, adding 263 days results in an estimated due date of July 27, 2025.
Ultrasound Scan
The most accurate method for estimating your due date, especially if you are uncertain about your LMP or conception date, is through an ultrasound scan. This ultrasound scan, done in the first trimester (around 7-13 weeks), measures the developing embryo or fetus's crown-rump length (CRL). The CRL measures the length of the developing baby from the top of its head (crown) to the bottom of its buttocks (rump).
Why Due Date Estimation Is Crucial for Prenatal Care and Pregnancy Planning
These are some key reasons why accurately determining your Estimated Due Date (EDD) is important.
- It helps your healthcare providers schedule important tests and visits at the right time (for example, the glucose screening at 24–28 weeks).
- It ensures your baby’s growth is monitored appropriately.
- It helps you track pregnancy milestones.
- It provides a framework for you and your family to plan for the baby's arrival (from preparing the nursery and arranging parental leave to even mental preparation).
- It also helps mitigate risks associated with preterm (birth before 37 weeks of pregnancy) and post-term (birth after 42 weeks of pregnancy) births [6].
Can My Due Date Change During Pregnancy?
Research has shown that Naegele’s rule generally calculates the estimated due date during pregnancy by adding nine months and seven days to the first day of the last menstrual period (LMP) [3]. While this is a handy starting point, it relies on knowing the exact date of your LMP and assumes the 28-day cycle length and day-14 ovulation timing, which is not always the case.
There are several reasons why healthcare providers may adjust your due date during pregnancy.
Early Ultrasound Accuracy: An early ultrasound, ideally between 7 and 13 weeks of gestation, is the most accurate way to date a pregnancy. During this ultrasound, measurements of the fetus, such as the crown-rump length (CRL), correlate very closely with gestational age. If the ultrasound measurements differ significantly (usually by more than 5-7 days) from the date calculated by your LMP, your healthcare provider will likely adjust your due date based on the more precise ultrasound findings.
Irregular Cycles or Unknown LMP: If your menstrual cycles are irregular or you cannot recall the exact date of your last period, the initial due date calculation based on LMP will be less reliable. In these situations, an early ultrasound becomes even more important for estimating a more accurate gestational age and due date.
Later Ultrasound Findings: While early ultrasounds are the usual standard for determining the length of gestation period, significant discrepancies found in later ultrasounds (second or third trimester) might sometimes prompt a due date adjustment, although this is less common. These later adjustments are usually considered if there are consistent and substantial differences in fetal growth measurements across multiple ultrasounds as the pregnancy timeline progresses [2].
Your initial due date is a preliminary calculation, often based on your last menstrual period. It is a good starting point for planning for your baby's arrival. However, an adjusted due date based on an early ultrasound is more accurate. This adjusted date provides a more reliable timeframe for monitoring your pregnancy's progress [2].
What Moms Say About Late Due Dates?
A study by Beth Lemmel Metz showed that over half of the respondents (53.74%) reported feeling stressed about when their baby would arrive. However, a significant majority (86.81%) did not feel it was unsafe for their baby to remain unborn at the end of their pregnancy. (9)
Another study conducted in 2017 on ten pregnant women and how they felt about their due dates, especially because they had post-term pregnancies. They described the experience as limbo, accompanied by negative feelings like restlessness, irritation, and frustration. Their thoughts became more negative as the days passed. Surprisingly, positive thoughts and feelings remained, including trust in their body’s ability to begin birth. (10)
The following statements were made by these women, eight of them first-time mothers, and the remaining two, second-time mothers:
- “You try to keep up your spirits anyway and convince yourself that everything is okay, but the emphasis is on the downside after the calculated date, so I feel that mentally it’s mainly negative.”
- “A feeling that nothing has happened, it will never come, this will continue day in and day out, and I will wake up like this every day in some sort of limbo.”
- “I try to jot down (activities) mainly so that I say that tomorrow, right, I have to do this, and it’s almost good that I won’t give birth tomorrow so that I can do this beforehand, constantly ticking things off that are super important.”
- “Just this not knowing...is something wrong with the unborn child because it doesn’t want to come out, or can something happen if it doesn’t come out?… I try not to think about that because it’s destructive…you really try to stop thinking about it, but the fact that nothing is happening is constantly at the back of your mind.”
- “The closer, yes, the closer it gets, the more worried I become…yes, because I begin to wonder; is there something wrong with me or is it because I don’t have that hormone and why don’t I have it and…will I need to be induced in all pregnancies?”
Frequently Asked Questions
What is a due date calculator?
A due date calculator is a tool that estimates the time of a baby's arrival, typically by adding approximately 280 days to the first day of the last menstrual period (LMP).
Is the due date 100% accurate?
No, the due date is not 100% accurate. As pregnancy timelines vary, it provides an estimated range rather than a definitive date.
What if I don't know the date of my last menstrual period?
If you don't know your LMP, an early ultrasound scan (around 7-13 weeks) is the most accurate method for estimating your due date.
Why is pregnancy calculated from my last period, not conception?
Pregnancy is commonly calculated from your last menstrual period because it is usually the first reliably known date for most women. Ovulation and conception dates can be less certain. Naegele's rule is based on this LMP start date.
Can ultrasound change my due date?
An early ultrasound (between 7 and 13 weeks) is the most accurate way to date a pregnancy. Healthcare providers will likely adjust your due date based on ultrasound measurements if they significantly differ (usually by more than 5-7 days) from the LMP-calculated date.
Can tracking my ovulation help estimate my due date more precisely?
Yes, actively tracking your ovulation and knowing your approximate conception date can help make a more personalized estimation. You can add 266 days to your believed conception date.
Can a baby be born before or after the estimated due date?
Yes, a baby can be born before the estimated due date (preterm, before 37 weeks) or after (post-term, after 42 weeks). The due date is an estimate within a natural range of variation, which is 39-40 weeks.
What if I have multiple pregnancies (twins, triplets)?
While you can use a standard due date calculator for an initial EDD in multiple pregnancies, the calculated date is unlikely to be the actual delivery date. Multiples arrive earlier than singletons, with "full term" often considered around 36-37 weeks for twins and even earlier for higher-order multiples. More than half of all twins are born preterm. Early ultrasounds will give more accurate gestational age assessment and provide a more realistic delivery timeline based on the specific multiple pregnancy. Therefore, consulting with your doctor is very important.
Can I use the due date calculator if I have irregular periods?
Yes, you can use a due date calculator with your LMP, but the estimation might be less accurate if you have irregular cycles. In this situation, fertility tracking or relying on an early ultrasound is recommended for a more precise estimate.
Can I use a due date calculator if I did IVF?
Yes, due date calculators can be very precise with IVF pregnancies. You can calculate your EDD by adding the gestational age of the embryo at transfer to the expected full gestation period (typically 266 days from egg retrieval for a day-5 transfer, or adjusted accordingly for the embryo's age at transfer).
When should I start using the due date calculator?
You can use a due date calculator as soon as you know the first day of your last menstrual period or have a confirmed conception date.
Can I trust the calculator for a cesarean section?
The due date calculator provides an estimation of when labor might naturally occur. The timing of a planned cesarean section is a medical decision you and your healthcare provider made based on various factors, not solely the estimated due date.