Due date is the estimated date your baby is expected to be born. However, babies rarely follow a strict schedule, so consider your due date as a guideline—not an exact answer. Most births occur between weeks 37 and 42 of pregnancy.

Although you can certainly calculate it with a calendar and some math, there’s something special about using an online due date calculator. It’s fast, efficient, and very concrete. The visual element also helps make everything feel more real.

Most due date calculators ask for the following:

• First day of your last period
• Alternatively: Date of ovulation or conception
• Cycle length (especially if it differs from the typical 28 days)

Once the information is entered, you’ll quickly get a due date — and often even a week-by-week overview of how far along you are. Suddenly the waiting becomes tangible and easier to relate to.

It feels as if the pregnancy already has day one in the system, and that often brings joy, anticipation, and plenty of imagination. Thoughts can already wander to all the first clothes, the future name, and what it will be like to meet a new little person.

The whole due date game is really a kind of guessing game — albeit a fairly informed one. Nature, as you know, can be a bit less predictable than an app or a calendar.

Statistics show that only 3–5% actually give birth on the calculated due date. The vast majority, however, deliver within a couple of weeks around the expected day, because it’s ultimately the body and baby who ‘decide’ when the time is right.

List of factors that can influence when labour starts:

• Genetic factors (family tendency)
• How many children you’ve previously had
• Specific pregnancy conditions (e.g., too much or too little amniotic fluid)
• Exact timing of fertilization
• The baby’s size and development

Although due date calculators provide a very good estimate, they are best used as a guideline — not a fixed contract.

If you know the date of your ovulation, the due date calculation can be even more accurate. Fertilization typically occurs within 24 hours after ovulation, and you add 38 weeks to the ovulation date.

If you want to calculate the due date from the date of intercourse, you still need to know the time of ovulation, since fertilization occurs shortly afterward. However, sperm can survive in the uterus for up to 3–5 days, so intercourse in the days leading up to ovulation can also result in pregnancy.

If you became pregnant through IVF, ICSI, or insemination, the calculation starts from the fertilization date and 38 weeks are added. This applies to both in vitro treatments and insemination.

Already at the first doctor's visit or ultrasound, many pregnant women report the first day of their last menstrual period. The doctor or midwife records this in the medical file and continues the tracking schedule from there.

But at the nuchal translucency scan in weeks 11–14, the baby's size is measured, and the estimated due date may sometimes be adjusted. If the fetus has grown slightly larger or smaller than the schedule indicated, a new due date is given.

It can of course feel confusing if you thought you had the date under control. But ultrasound measurements provide a picture of exactly how far along the baby is in development, and therefore many receive an updated due date in connection with the nuchal translucency scan, and that is the date considered the most accurate.

The joy of anticipation can be hard to hold onto amid nausea and fatigue, but having a due date is something many pregnant people return to again and again. It frames the family’s plans for the coming period.

The due date is often the first day that gets marked in the calendar – and it’s the date family, friends, and even employers ask about.

The time leading up to the big day is spent:

• Planning parental leave and vacations
• Setting up the nursery
• Buying practical items and making wish lists
• Mental and perhaps physical preparation for the birth

Because the baby almost never arrives exactly on that day, the waiting period can feel like having Christmas Eve on a schedule where the date can shift. It’s simultaneously beautiful, frustrating, and kind of magical.

Your due date is 40 weeks (280 days) after the first day of your last menstrual period, or 38 weeks (266 days) after ovulation or fertilisation. It’s completely normal for the due date to be adjusted along the way if a scan shows the baby is larger or smaller than expected. Learn more about your due date system here.

If your menstrual cycle is irregular, or for other reasons you don’t know the start date of your most recent bleeding, an early ultrasound scan can be used to determine your due date. Any adjustment of the due date is usually made in connection with the nuchal translucency scan around week 12.

Do you also enjoy following your baby’s development week by week? A pregnancy calendar can help you keep track of how far along you are. As with calculating the due date, you start from the first day of bleeding in your most recent period.

Many people think they are 0 weeks and 0 days pregnant at the moment of conception – but pregnancy isn’t actually counted that way.

Fertilization typically occurs around 14 days after the first day of your last period. That means you are often about 4 weeks pregnant when your period is late and you discover you are expecting.

A pregnancy lasts about 40 weeks, which we commonly refer to as 9 months and divide into three trimesters. This time is counted from the last menstrual period. If you look at the time from fertilization itself, the pregnancy lasts about 38 weeks (266 days).

To make it easier to follow the development of your body and baby, pregnancy is divided into three trimesters. Each trimester has its own changes and milestones. Read more about each trimester here.

First trimester
1st month: weeks 1–4
2nd month: weeks 5–8
3rd month: weeks 9–13

Second trimester
4th month: weeks 14–17
5th month: weeks 18–22
6th month: weeks 23–27

Third trimester
7th month: weeks 28–31
8th month: weeks 32–35
9th month: weeks 36–40

Only about 4–5% give birth on the actual due date. The due date should therefore be seen as a guideline rather than an exact day. In Denmark, most babies are born around 40 weeks and 2 days, that is typically after the due date.

The timing of a planned cesarean depends on the reason. If neither the mother nor the baby is at risk, the procedure is usually scheduled as close to the due date as possible.

It is generally safe to go a few days past your due date as long as both you and the baby are doing well. Once you reach 41+0 weeks, you will be offered extra monitoring, and it is usually recommended to induce labor before 42 full weeks.

During these check-ups, a midwife or doctor monitors the baby’s well-being, movements, heart rate, and the amount of amniotic fluid.

Being born after week 40 is rarely harmful, but there can be an increased risk of reduced amniotic fluid or lack of oxygen during birth, which is why you will be followed more closely.

There are many myths, but little suggests that the baby’s sex or your age matters. Research instead points to these factors:

• It’s your first baby (about 28% go past term)
• Heredity on your mother’s side
• The due date is estimated imprecisely
• Severe overweight
• You have previously gone past your due date
  

Pregnant people with multiples often give birth before week 37. Women who have given birth before may also tend to deliver a little earlier. In addition, stress, smoking and certain medical conditions can play a role.

In some cases, labor is induced early for the sake of the mother or baby.

If you are expecting twins, pregnancy rarely lasts beyond 38 weeks. Many hospitals recommend induction around week 38 if labor hasn’t started, to ensure the best outcome for both babies.