How to Use Our Pregnancy Calculator
To calculate your estimated due date and pregnancy timeline, we need a starting point. You can use one of two methods below.
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First Day of Last Period (LMP): This is the most common method. Select the date the first day of your last menstrual period began. Even though conception happens about two weeks later, doctors use this date as the official starting point for your 40-week pregnancy term.
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Average Cycle Length: Enter the typical number of days from the start of one period to the start of the next (e.g., 28 days, 30 days). The standard is 28 days, but if your cycle is longer or shorter, providing this number allows for a more accurate calculation of your ovulation and conception date.
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Conception Date (Optional): If you know the exact date of conception (for example, through ovulation tracking or fertility treatments like IUI or IVF), you can use this option for a more precise result. Using this method will calculate your due date based on a 38-week gestation period from conception.
Understanding Your Results
Your results provide more than just a single date; they offer a snapshot of your entire pregnancy timeline. Here’s a breakdown of what each part of your result means.
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Estimated Due Date: This is the date your baby is predicted to arrive. It is calculated by adding 40 weeks (280 days) to the first day of your last menstrual period (this is known as Naegele’s Rule). Remember, this is an estimate—only about 5% of babies are born on their exact due date. A normal, full-term delivery can happen anytime between 38 and 42 weeks.
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You Are… (Gestational Age): This tells you how far along you are in your pregnancy, shown in weeks and days. This is your “gestational age,” which is the standard measurement used by healthcare providers. It’s measured from your last menstrual period (LMP), not from the date of conception. This is why you are considered “two weeks pregnant” at the time of conception.
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Estimated Conception Date: This is the approximate date you likely conceived. It’s calculated based on your cycle length and LMP, typically occurring around 14 days after your period started for a 28-day cycle.
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Fetal Age: This is the actual age of your growing baby since the moment of conception. Fetal age will be about two weeks less than your gestational age.
Your Personalized Pregnancy Timeline
Based on your due date, here are the key milestones and phases of your pregnancy journey.
Frequently Asked Questions
What if I have irregular periods? How does that affect my due date?
If your cycles are irregular or you don’t know the date of your last period, a due date calculator will be less accurate. Irregular cycles make it difficult to pinpoint the date of ovulation.
In this situation, the most reliable way to determine your due date is with a first-trimester ultrasound. A healthcare provider will measure the baby from crown to rump (Crown-Rump Length or CRL). This measurement is highly accurate in the early weeks and will give you the most precise gestational age and due date.
Why is my doctor’s due date different from the one the calculator gave me?
It’s very common for a due date to be adjusted after your first prenatal appointment. While this calculator provides a great estimate based on the information you provide, your doctor will use an ultrasound scan to date the pregnancy.
The first-trimester ultrasound is considered the gold standard for dating. If the date from the ultrasound is significantly different from your LMP-calculated date (usually by more than 5-7 days), your doctor will likely use the ultrasound date as your official estimated due to date.
How accurate is an estimated due date?
Think of a due date as a “due week.” It’s a helpful guideline, but very few babies are born on their exact due date. A full-term pregnancy can last anywhere from 38 to 42 weeks. It is perfectly normal for your baby to arrive a week or two before or after your estimated due date. The date is a target, not a deadline.
What are the trimesters of pregnancy?
Pregnancy is divided into three “trimesters,” each lasting about 13-14 weeks. Each one marks a distinct phase of fetal development and maternal changes.
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First Trimester (Weeks 1-13): This is the most critical period for your baby’s development. All major organs and systems are formed. For you, this trimester is often marked by symptoms like fatigue, nausea (“morning sickness”), and hormonal changes.
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Second Trimester (Weeks 14-27): Many women find this to be the easiest trimester. Early symptoms often fade, energy levels may increase, and you’ll likely start to show and feel your baby move.
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Third Trimester (Weeks 28-40+): This is the final stretch. The baby gains the most weight during this time, and your body prepares for labor and delivery. You may experience more physical discomforts like backaches, frequent urination, and swelling as the baby gets bigger.
Is it possible to get pregnant right after my period ends?
While it’s less likely, it is possible. Your “fertile window” is the time in your cycle when you can conceive—typically the five days leading up to ovulation and the day of ovulation itself. If you have a very short menstrual cycle (e.g., 21-22 days), you could ovulate just a few days after your period ends. Since sperm can survive in the female reproductive tract for up to five days, having intercourse immediately after your period could lead to pregnancy if you ovulate soon after.
Next Steps for Your Pregnancy Journey
Planning is an exciting part of expecting. Use our other tools to help you prepare for what’s ahead.
