Pregnancy Due Date Calculator
Calculate your estimated due date using your last menstrual period, conception date, or ultrasound results.
How Is Your Due Date Calculated?
The Last Menstrual Period (LMP) Method
The most common and standard method for estimating a due date is based on the first day of your last menstrual period (LMP). It's a straightforward calculation used by healthcare providers worldwide.
Your Due Date = First Day of Your LMP + 280 days (or 40 weeks)
This method works because it uses a concrete date that is usually known. It operates on a key assumption: that you have a regular 28-day menstrual cycle and that ovulation (when you likely conceived) occurred around day 14.
The Ultrasound Method
During the scan, the doctor or sonographer measures the baby from head to bottom. This is called the Crown-Rump Length (CRL). The ultrasound machine's software then uses this measurement to determine the precise gestational age of the fetus. For example, it might determine the age is "8 weeks and 4 days."
Once the exact age is known, the due date is calculated by projecting forward to the 40-week mark. The formula is:
Your Due Date = Date of Ultrasound - Gestational Age at time of Ultrasound + 280 days (or 40 weeks)
The Conception Date Method
If you know the exact date you conceived—often through methods like ovulation tracking or fertility treatments such as IVF—this method can provide a highly accurate due date.
The calculation is the most direct of all the methods, as it uses the actual start of the 38-week gestational period.
Your Due Date = Date of Conception + 266 days (or 38 weeks)
You might notice this is 38 weeks, not the 40 weeks used in the LMP method. This is because the Conception Date method starts from the true beginning of pregnancy, whereas the LMP method includes the approximately two weeks leading up to ovulation.
Frequently Asked Questions
What does the due date mean?
The due date is an estimate of when your baby will be born. It takes you to the 40 week mark of your pregnancy. A full-term baby can be born anywhere between 37 and 42 weeks. Over 41 weeks is classed as overdue, but it isn't abnormal. Only 4% of babies are born on their estimated due date.
Which method of calculating the due date is most accurate?
While the LMP method provides an initial estimate, ultrasound dating, particularly in the first trimester, is the most accurate method for determining a due date. If a woman has a regular cycle and accurately remembers her LMP, the two methods might align. However, in cases of irregular cycles or uncertainty about the LMP, an early ultrasound is recommended for a more precise estimation of the due date.
Why is the due date calculated from the last menstrual period?
Pregnancy is typically considered to be 40 weeks long. Interestingly, this countdown doesn’t start from the day of conception or ovulation, but from the first day of your last menstrual period (LMP). The LMP is used as a reference point because it’s a clearly identifiable and reliable date for most women, even though actual conception usually occurs about two weeks later.
Can I calculate my due date if I don't know my last menstrual period?
Yes, you can still estimate your due date even if you don't know the date of your last menstrual period. In this case, an early ultrasound can help determine how far along you are by measuring the size of the baby. This method is especially useful if your periods are irregular or hard to track. Your healthcare provider will use the ultrasound measurements to give you the most accurate due date possible.
Are due date calculators accurate?
Due date calculators offer an estimate based on the average length of pregnancy and the details you provide. While they’re a helpful tool, it's important to remember that every pregnancy is different. Many factors can influence when labor actually begins, and only about 4% of babies are born on their estimated due date. Think of it as a general guide rather than a set-in-stone prediction. In the end, your baby decides when they're ready — your job is to be as prepared as possible for that moment.
Are first babies usually late?
It's a common belief that first babies tend to arrive later than expected, and studies support this idea. First-time mothers often experience longer pregnancies, with many delivering after their due date. However, every pregnancy is unique, and while some first babies may be late, others can arrive early or right on time. It's essential to stay flexible and prepared for any scenario as your due date approaches.