Estimated Due Date Calculator (EDD) | Oh Mama Matrona
When you get pregnant and share the happy news, everyone asks you how far along you are. This article explains how to calculate your estimated due date and understand your pregnancy timeline.
When you get pregnant and share the happy news, absolutely everyone asks you, "How far along are you?"
In every single appointment with your midwife or gynecologist, they tell you that you are "x" weeks along... And yes, I can imagine your face short-circuiting because you don't quite know how to translate weeks into months. And believe me, we all go through that.
The estimated due date calculator is the quickest way to answer that question in a language your interlocutors understand.
But beware, if the question about the months of pregnancy was the first one they ask you, the second will probably be, "When is your due date?" As if you had a crystal ball that could predict the exact day your little one will decide to come into this world!
As I am convinced that you are the first interested in knowing in which month your baby might arrive, I have good news for you: a tool developed by our team that can answer both questions. It will help you know where you are in your pregnancy and how much longer you have until you give birth.
Estimated Due Date Calculator (EDD)
It’s very easy: enter the first day of your last menstrual period in the form and let the calculator do its magic.
How does the estimated due date calculator work?
The basis of the calculation is the same as what we use in consultations. You only need one date: the first day of your last menstrual period.
You might think... why does my midwife need to know the date of my last period if now that I’m pregnant I don’t expect her to visit for a long time?
It’s very simple, let me explain:
The first day of your last period
In the estimated due date calculator, we take the date of your last period (LMP) because it is the one that marks the start of the cycle. From there, ovulation and fertilization occurred in the following days, but... we don’t know the exact day when that egg and sperm met and fertilization happened.
That’s why all the estimates start here, on the date we do know for sure: the day your menstrual cycle began with the start of your period. From that day, weeks of pregnancy are counted (including those 2 weeks prior to fertilization "in blank").
Although it may seem nonsensical, it is the most practical and widely used way to count weeks of pregnancy.
Why add 280 days?
When we add 280 days, we are using the average duration of pregnancy. It’s a standard that works well for most women, although each pregnancy follows its own rhythm.
When we add 280 days, does that mean my pregnancy will last exactly 280 days, not one more or one less?
Absolutely “No”. It’s a standard that works well for most women, but each pregnancy follows its own rhythm. Some babies decide to come into this world a little earlier, while others are a bit lazier, taking a few more days in the comfort of the womb.
Later on, adjustments may be made if your first-trimester ultrasound indicates a more precise date.
What if my cycles are irregular?
No problem. The calculator gives you an initial estimate, and if your cycles are very irregular or you don’t quite remember the date of your last period, the first-trimester ultrasound is what ultimately establishes the date more accurately. This is the usual procedure.
You might be thinking that this article doesn’t apply to you because your cycles are irregular and you’re not sure if you ovulated on day 14 of the cycle. As is often generalized...
No problem. The calculator gives you an initial estimate, and if your cycles are very irregular or you don’t quite remember the date of your last period, medicine also has a solution for you: the first-trimester ultrasound is what ultimately establishes the date more accurately. This is the usual procedure.
How many months am I?
Oh, friend. This is one of the most frequently asked questions (and most searched on the internet): “How many months is X weeks?”.
And here comes the moment of truth: there is no official equivalence in medicine because months have different durations.
This answer might disappoint you, so I bring good news to make up for it. In my book Oh Motherhood, I use a table that helps families a lot. It’s the same one used by the calculator you just saw.
Table of equivalence between weeks and months of the calculator
Remember that the estimated due date calculator is indicative, not an exact anatomical or physiological measurement. It’s simply a way to help understand what month of pregnancy you are in:
- Month 1: from 1 to 4 weeks
- Month 2: from 5 to 8 weeks
- Month 3: from 9 to 13 weeks
- Month 4: from 14 to 17 weeks
- Month 5: from 18 to 22 weeks
- Month 6: from 23 to 27 weeks
- Month 7: from 28 to 31 weeks
- Month 8: from 32 to 35 weeks
- Month 9: from 36 to 40 weeks
- Month 10: from 41 to 42 weeks
If you want more personalized follow-up, you can request a pregnancy follow-up consultation, where the midwives of the team, and I myself, take care of guiding you throughout the motherhood process.
Thanks to them, you will not only “not be left with a poker face” after “The question,” but you will also understand, appointment by appointment, what is happening in your body and how you can enjoy pregnancy without fears or guilt.
Examples of what month corresponds to your weeks
To give you an idea:
- 20 weeks: month 5
- 27 weeks: month 6
- 33 weeks: month 8
- 36 weeks: month 9
And yes, there is the famous “10th month” of pregnancy. It refers to weeks 41 and 42, and it doesn’t mean that the baby is delayed; rather, just as not all people are the same height, not all pregnancies last exactly the same.
And remember something very important, a pregnant woman is not yogurt, so she doesn’t expire. Your pregnancy doesn’t have a date after which you have “overdue.”
What is the purpose of knowing your estimated due date?
Knowing your estimated due date places you in your pregnancy and allows you to anticipate what’s coming. With that reference, we organize ultrasounds, blood tests, and follow-up visits, and we also interpret the symptoms of each stage better. A nausea in week 7 is not the same as in week 28.
As each phase of pregnancy has its changes, knowing the exact week helps us check that everything is progressing as expected, and it’s much easier to make sense of what you feel.
The EDD also guides your calendar. For example, appointments with the midwife, childbirth preparation, or even family organization.
Can my estimated due date change?
As the pregnancy progresses, it’s normal to have doubts about whether the estimated date will remain the same. So let’s see what can influence this figure to adjust slightly:
The first-trimester ultrasound and gestational dating
The first-trimester ultrasound is more accurate than the LMP for dating the pregnancy.
Do you remember what we mentioned at the beginning about how not all women ovulate on day 14 of the menstrual cycle?
Well, precisely to avoid that being a problem, the first-trimester ultrasound is used; to confirm or modify how many weeks you are. It allows measuring the embryo with very high accuracy and, therefore, can adjust the EDD.
In fact, important guidelines like those from the American College of Obstetricians and Gynecologists support this method as one of the most reliable.
Why does my midwife sometimes update the EDD?
If the LMP does not match the ultrasound measurements, it is common for the professional to choose the most accurate estimate. This helps interpret the development of the pregnancy correctly, adjust the diagnostic tests to the correct dates, and prepare for the birth.
How long does a pregnancy really last?
Traditionally, people talk about “nine months,” but the truth is that a pregnancy usually lasts between 37 and 42 weeks. Therefore, it’s completely normal for your baby to arrive a few days before or after week 40.
It’s just as normal to give birth in week 38 as in week 41. And sometimes, not knowing that week 40 is by no means the end of the pregnancy takes the pressure off when we reach, or even surpass, that date, and there are no signs that the little one wants to come out.
Term, post-term, and prolonged pregnancy
A pregnancy is considered term between weeks 37 and 42, preterm before 37 weeks, and prolonged beyond week 42.
None of these is a cause for alarm, but when reaching week 42, induction of labor is usually offered, which you can accept or decline.
If you decline the offer, it’s important to monitor the pregnancy with successive visits every few days. To ensure that continuing with the pregnancy does not pose a risk to you or the little one.
Why do we talk in weeks and not in months?
You might think that healthcare providers complicate things by measuring pregnancy in weeks and not in months, but this way, we can track the pregnancy more accurately.
Every week something different happens: hormonal changes, baby growth, organ development… Using weeks helps communicate better what is expected at each phase and what tests are due at each moment.
Frequently asked questions about pregnancy calculation
Here are some of the questions I often hear in consultations that can help you better understand the calculations of pregnancy duration and the estimated due date:
What happens if I reach week 41 or 42?
It’s common to perform more frequent checks to ensure that both you and the baby are well. It doesn’t mean there’s a problem, but it does require closer supervision.
What if I don’t remember my last period?
Don’t worry (it happens more often than you think). The first-trimester ultrasound is the best tool for estimating gestational age when the LMP is unclear.
Can I calculate the conception date?
With the estimated due date calculator, you can only make approximate connections. Fertilization usually occurs between 12 and 16 days after the start of the cycle in women with regular cycles, but since women are not machines, this is not an exact science.
Is it reliable to calculate pregnancy with apps or online wheels?
As a guideline, yes. But it obviously doesn’t exclude that you make a visit to your midwife or healthcare professional, who will give you the definitive figure based on your ultrasounds and medical history.
Why don’t all women give birth in week 40?
Pregnancy doesn’t have an ON-OFF switch. We can’t press a button to make it end.
Every woman is different, and her pregnancy follows its own rhythm. Only a small percentage of babies are born exactly on the estimated due date (about 4%, in which, interestingly, I find myself as a baby who arrived in this world on the exact estimated due date given to my mother). But the reality is that most do so within the completely normal range of between weeks 37 and 42.
The accuracy of the estimated due date
It is estimated that only between 4% and 8% of babies are born on their estimated due date (EDD). However, knowing it is important to have an idea of when you might give birth; it allows you to better organize medical follow-ups and monitor your baby’s development.
Thank you for trusting this tool and my work. And remember, your pregnancy is unique, and accompanying you through it is always a privilege.
What to do after knowing your estimated due date
Once you have your estimated due date...
