Most parents want to know exactly when they can expect the arrival of their little explorer—but it’s not always easy to predict. In this article, you’ll learn why a pregnancy doesn’t necessarily last exactly nine months, how your baby’s estimated due date is determined, and why it isn’t a fixed date.

Why is the estimated due date important?

The estimated due date (also ET or EGT ) helps expectant parents prepare for a specific time frame and gives you a rough idea of when your baby will be born. Determining the current week of pregnancy also provides information about which Preventive examination is coming up. Your doctor can assess the course of the pregnancy and your baby’s age-appropriate development based on the calculated due date. That’s why the due date, together with the results of the respective examinations, is recorded in your Mother's Pass . The estimated due date is also important for your employer. It determines the start of your Maternity protection , which begins six weeks before the due date and ends eight weeks after the birth. If the birth occurs earlier than planned, the protection period is extended by the corresponding number of days. Conversely, maternity protection does not shorten if your baby is born after the estimated date.

SSW Calculator: Calculate Due Date & Pregnancy Week

How is the estimated due date determined?

Various methods are used to determine the due date. We’ve summarized the different options, along with their advantages and disadvantages, for you:

1. Mathematical calculation(s)

• Counting method from fertilization

A pregnancy begins with the fertilization of the egg in the middle of the female cycle, approximately 12 to 16 days before the theoretical start of the period. If you know the exact day of fertilization, you can determine the due date using the so-called Conceptual method :

Appointment date minus 3 months minus 7 days plus 1 year

From the time of fertilization, the duration of pregnancy is 266 to 267 days, or 38 weeks. The prerequisite for this due date calculation method is knowing the exact day of fertilization. However, it’s often not possible to determine this with certainty, as many women have an irregular cycle or ovulation can shift depending on various factors.

• Counting method from the last period

For many women, it’s therefore easier to identify the first day of the last period and calculate the due date based on that. This can be done using the so-called Naegele's rule :

First day of the period plus 1 year minus 3 months plus 7 days (plus/minus the number of days the cycle length deviates from the 28-day "normal rhythm")

Calculated from the time of your last period, a pregnancy lasts 280 to 282 days, or 40 weeks, although you are not actually pregnant during the first two weeks. Therefore, there is always approximately a two-week difference between the age of your unborn baby and the pregnancy week recorded in the maternity record. The prerequisite for determining the due date using Naegele’s rule is that you have a regular cycle and know when your last period was.

Both calculations provide approximate values, as they do not account for the varying lengths of the months. Both also assume that a cycle lasts exactly 28 days, which is not always the case. With Naegele’s rule, this inaccuracy can at least be compensated for in the calculation.

2. Calculation with the Gravidarium

How are pregnancy weeks counted?

As you can see, there isn’t just one way to define the duration and age of a pregnancy. For the sake of a uniform definition, in the European region a pregnancy officially begins from a medical perspective on the first day of the last period, which is about 14 to 16 days before actual fertilization.

From the beginning of your last period, an approximate pregnancy length of 40 weeks, or 280 to 282 days, is assumed. This corresponds to 10 lunar months, each exactly 28 days long, or nine calendar months, which are longer than 28 days—except for February. The question of whether a pregnancy lasts nine or ten months, which often causes confusion, is therefore a matter of interpretation and depends on whether you count lunar or calendar months.

The progress of a pregnancy is counted in completed weeks plus the number of days in the current week. For example, in week 25+5 of pregnancy, you have been pregnant for 25 weeks and 5 days, and you are in the 26th week of pregnancy. Additionally, pregnancy is divided into three trimesters, each lasting approximately 13 weeks or three months:

• First trimester: 1st to 12th week of pregnancy

• Second trimester: 13th to 28th week of pregnancy

• Third trimester: 29th week of pregnancy until birth

Good to know: How exactly is the due date calculated?

It can be reassuring to know when to expect the arrival of a new family member. However, it’s never possible to predict a child’s birthday with absolute certainty, and only four percent of all babies are born on their estimated due date. Every pregnancy is different, and its duration is influenced by factors such as the child’s development, and the mother’s age and health. Medically, the due date refers to the period from three weeks before to two weeks after the due date. So don’t let the date unsettle you; instead, prepare for the time around it, when your baby is likely—just like 88 percent of children—to see the light of day.

The estimated due date may rarely be your baby’s exact birthday, but it helps you know roughly when to expect your little explorer to arrive. In addition, the due date is an important guideline for prenatal checkups and the start of maternity leave. Although there are various methods of calculation, the (ultrasound) examinations with your doctor provide the most accurate result when it comes to knowing when you’ll hold your baby in your arms.

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