Calculate pregnancy week and due date
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 specific date.
Why is the estimated due date important?
The estimated due date (also ET or EGT ) allows expectant parents to prepare for a specific time and at least roughly know when their baby will be born. Determining the current pregnancy week also provides information about when which Preventive examination upcoming. Your doctor can assess the course of the pregnancy and the age-appropriate development of your baby based on the calculated due date. Therefore, the due date, together with the results of the respective examinations, is included in your Mother's Pass registered. The estimated due date is also important for your employer. It determines your start date in the 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, the 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 have summarized the different options along with their advantages and disadvantages for you:
1. Mathematical calculation(s)
-
Counting method from fertilization
A pregnancy always 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 calculate:
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 that you know the exact day of fertilization. However, it is often not possible to determine this with certainty, as many women have an irregular cycle or ovulation, depending on various factors, can sometimes shift.
-
Counting method from the last period
For many women, it is therefore easier to determine the first day of their last period and calculate the due date based on that. This can be done with 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 the Naegele 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 are also based on the assumption that a cycle lasts exactly 28 days, which is not always the case. At least with the Naegele rule, this inaccuracy in the calculation can be compensated for.
2. Calculation with the Gravidarium
The gravidarium is a rotating date wheel that is often used by midwives as a tool to determine the due date. It is based on the same principle as mathematical calculation, but it spares you the complicated math. Since all relevant data such as cycle length or the day of presumed fertilization can be set on the gravidarium, it provides instant information about the estimated due date and the probable birth period. Some gravidariums also offer an overview of when certain examinations are scheduled.
3. Calculation using ultrasound
A more precise determination of the due date is usually possible during the first prenatal check-up between the 5th and 12th week of pregnancy. Your doctor measures the size of your baby (crown-rump length, short: CRL) during the ultrasound. By comparing this with statistical growth values, they can determine the current week of pregnancy and the due date. The estimation of the due date via ultrasound is considered particularly reliable at the beginning of pregnancy, as all embryos tend to grow at approximately the same rate during this early phase. If the due date has already been calculated using other methods, it may be adjusted forward or backward based on the ultrasound.
During the upcoming ultrasound examinations, your baby's size will also be compared to average values for the current week of pregnancy, and the due date will be adjusted if necessary. Since the growth curve of babies varies more individually after the , the determination becomes increasingly less accurate as the pregnancy progresses.
How are pregnancy weeks counted?
As you can see, there is not just one way to define the duration and age of a pregnancy. In terms of a uniform definition, a pregnancy officially begins from a medical perspective in the European region 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 the new family member. However, it is 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 individually determined by factors such as the development of the child, the age, and the health of the mother. Medically, the due date refers to the period from three weeks before to two weeks after the due date. Therefore, do not be unsettled by the date, but rather prepare for the time around it, when your baby is likely, just like 88 percent of children, to see the light of day.
Pregnancy test
Your surroundings are probably eagerly awaiting the estimated due date as well. If you don't want to be bombarded with questions about whether your baby has arrived around the due date, a little cheating is allowed. ;) Simply push the estimated date a bit further or keep it vague in front of family and friends. The surprise will be all the more delightful!
The estimated due date may only rarely be the exact birthday of your baby, but it is helpful so you know approximately when to expect your little explorer to arrive. Additionally, the due date is an important guideline when it comes to prenatal checkups and the start of maternity leave. Although there are various possible methods of calculation, the (ultrasound) examinations with your doctor provide the most accurate result when it comes to knowing when you will hold your baby in your arms.
Do you want information tailored to your pregnancy stage? Then secure a reliable companion with our #momlife newsletter and receive valuable info and expert tips every two weeks, relevant to your current week of pregnancy, directly in your email inbox. The will be sent to you directly to your home.