What is a due date calculator?
A due date calculator shows the expected delivery date (due date) based on the date your last menstruation started. The calculation is based on an average pregnancy length of 280 days (40 weeks). This is the same method that Danish maternity wards and scanning clinics use as a starting point until a nuchal fold or malformation scan possibly adjusts the date.
See also: Pregnancy test at the general practitioner
How to use the calculator
- Select the first day of your last menstrual period on the calendar.
- Press “Calculate due date”.
- See your expected due date, week of pregnancy and today's status immediately (e.g. "You are in week 12 + 3").
The calculation is anonymous – we do not save dates or personal information.
What does week xx + x mean?
The notation "week 12 + 3" means that the pregnancy has lasted 12 full weeks plus 3 extra days. The first number indicates the number of full, completed weeks, while the number after the plus sign shows how many days have passed beyond the full weeks.
The system makes it easy for healthcare professionals to specify the fetus's precise age and the timing of examinations, because even a few days' difference can be important for scans, blood tests and assessment of development.
When are you due to give birth?
Although the due date provides a good benchmark, only about 4% of pregnant women give birth exactly on the calculated day. Most births occur in the window from 37 + 0 to 41 + 6 weeks, when the baby is considered full term. Your baby ultimately determines the timing via complex hormonal signals that start the contractions, and factors such as first-time birth, genetic predisposition, and your own cycle length can shift the timing a few days to either side. If you go more than 41–42 weeks, the hospital will typically offer induction to reduce the risk of complications. Therefore, use the due date as a guide, but have your bag ready a few weeks before – and enjoy the peace for the final preparations while you wait for your baby to choose their perfect birthday.
Scientific background
The length of pregnancy is measured from the first day of the last menstrual period (LMP). The reason is that ovulation is rarely determined precisely without blood tests or ultrasound. A cycle of 28 days places ovulation around day 14, and from there it is approximately 266 days to birth. By adding a full 280 days to LMP, you arrive at the estimated due date. Individual variations occur, and an ultrasound scan in the first trimester can move the date by a few days.
Pregnancy – body and mind in change
Pregnancy is a unique journey where both body and mind undergo significant changes – from the initial hormonal fluctuations that can cause fatigue and nausea to the growing belly that reminds you of the little life inside. Many experience increased emotional vulnerability, but also a deep connection with the child and partner.
Give yourself space to rest, eat nutritious food, and engage in light exercise such as walks or pregnancy yoga; it strengthens both circulation, mood and sleep. Remember that each pregnancy is unique: listen to your body and seek advice from your midwife or doctor if you are unsure about symptoms or need extra support.
Read more articles about pregnancy here.
Frequently Asked Questions
I don't know the date of my last menstruation. What do I do?
Use your best guess. The date can be adjusted when you get your first scan.
My cycle is longer/shorter than 28 days – does this affect the due date?
Yes. For example, if your cycle is 32 days, you ovulate approximately on day 18; adjust by adding the extra number of days before using the calculator.
The calculator shows a different due date than my scan. What applies?
Ultrasound in the first trimester is more accurate than the LMP method. Always follow the instructions of your midwife or doctor.
