Pregnancy Due Date Calculator
Wondering when your baby is due? Our free due date calculator gives you an instant estimate based on your last menstrual period or conception date plus how many weeks pregnant you are right now and which trimester you’re in.
How Does a Due Date Calculator Work?
Most pregnancy due date calculators use Naegele’s Rule, a simple formula that adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes an average 28-day cycle, with ovulation happening around day 14. If your cycle is shorter or longer than 28 days, the calculation shifts slightly — which is why our calculator lets you enter your average cycle length for a more personalised estimate. If you know your conception date (for example, from IVF or ovulation tracking), we add 266 days instead, since this counts from ovulation rather than the start of your period.
How Accurate Is a Due Date Calculator?
It’s important to set realistic expectations: a due date is an estimate, not a guarantee. According to NHS guidance, only around 1 in 20 babies are born on their actual due date. Most babies arrive within a window of two weeks before or after, and this is completely normal.
Your midwife or sonographer will usually confirm (or slightly adjust) your due date at your dating scan, typically offered between 10 and 14 weeks. This scan measures your baby and is generally considered more accurate than a calculation based on your last period alone, especially if your cycles are irregular.
Understanding the Trimesters
Pregnancy is generally divided into three trimesters:
First trimester: Weeks 1–12: Early development, and when common symptoms like morning sickness often begin.
Second trimester: Weeks 13–26: Often called the “golden period,” when energy levels tend to improve.
Third trimester: Weeks 27–40: The final stretch, as your body prepares for labour and birth.
Frequently Asked Questions
Can a due date calculator be wrong?
Yes. It provides an estimate based on average cycle lengths and typical pregnancy duration. Your actual due date can shift based on your dating scan results, especially if you have irregular cycles or aren’t sure of your last period date.
What if I don’t know my last period date?
If you’re unsure of your LMP, use the conception date option instead, or wait for your dating scan, which uses ultrasound measurements to estimate your baby’s age.
How many weeks is a full-term pregnancy?
A pregnancy is considered full term between 37 and 42 weeks. Babies born before 37 weeks are considered premature.
Does the due date change after my scan?
Sometimes, yes. If the scan measurements suggest your baby is measuring differently than expected from your LMP, your midwife may adjust your due date slightly.