Duration of pregnancy

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Classification according to ICD-10
O09.0! Less than 5 completed weeks
O09.1! 5 to 13 completed weeks
O09.2! 14th week to 19 completed weeks
O09.3! 20th week to 25 completed weeks
O09.4! 26th week to 33 completed weeks
O09.5! 34th week to 36 completed weeks
O09.6! 37th week to 41 completed weeks
O09.7! More than 41 completed weeks
O09.9! Unspecified (in the above classifications, the duration of pregnancy is the time from the first day of the last normal menstruation)
ICD-10 online (WHO version 2019)

The duration of pregnancy , also gestation period or gestational age , describes the period of pregnancy . It is usually given from the first day of the last normal menstruation. The actual (real) gestation period is the time from fertilization . According to German criminal law, pregnancy does not begin until the time of implantation .

The knowledge of the normal duration of pregnancy is used, among other things, to determine the calculated date of birth , abbreviated EGT , also calculated date , abbreviated ET . In humans, births within the three weeks before and two weeks after the calculated date are also considered to be "on the date" or "on time".

Basics and definitions

A woman's pregnancy, from conception to birth, takes an average of nine months, more precisely 268 days, i.e. 38 weeks plus 2 days, with a fluctuation range of around 5 weeks.

Because the exact time of fertilization (i.e. the day on which the sperm and egg cells fuse ) is often not known, two differently defined timescales have become established. Either count from the day of conception (p. C. = Post conceptionem) or from the first day of the last period (p. M. = Post menstruationem). A conversion is possible because fertilization takes place approx. 14 days after the last menstruation, but a mechanical conversion (p. M. Minus 14 is p. C.) Leads to inaccuracies.

The individual days are named according to a code that contains the number of weeks and the number of additional days. "37W3" means 37 weeks and 3 days.

post conceptionem (pc)

This term comes from the Latin ( post "after" and conceptio "conception") and describes the duration of a pregnancy from the day of conception ( impregnation ). The gestational age is given in completed development weeks and days (p. C.), For example in the legal regulations on termination of pregnancy in Germany (§ 218a StGB).

post menstruation (pm)

This term is also of Latin origin ( menstruare “to have the monthly flow”, to menstruus “monthly”) and describes the duration of a pregnancy from the first day of the last menstrual period ( menstruation ). This invoice is used in maternity passports and in the clinic and is also given in completed weeks and days of pregnancy. Here, a cycle duration of 28 days is assumed, fertilization occurs around the 14th day of the cycle. According to this definition, an average pregnancy lasts 280 days, i.e. 40 weeks. This calculation therefore includes the approximate two weeks before the impregnation . (Individual differences in cycle duration can be taken into account with the extended Naegele rule , see below.)

Methods for determining the age of gestation

The actual start of labor depends on many factors such as the age of the mother, whether a single or multiple pregnancy is present, and external circumstances. In practice, the following two simple methods are used to determine the average end of pregnancy.


The Naegele rule is used to calculate the average end of pregnancy depending on the start of the last period: from the first day of the last period plus 280 days or from the first day of the last period plus one year minus three months plus seven days. (Pregnancy duration 40 weeks). The extended Naegele rule also includes the cycle duration if it regularly differs from the average 28 days.

The resulting date corresponds to the calculated due date (EGT). According to the statistics, the highest percentage of children is born on the calculated day (5.5 percent , see below), but this also means that 94.5 percent of children are not born on the calculated date .

Ultrasound examination (sonography)

During the ultrasound examination, some body structures of the embryo or fetus are measured, such as the total length (crown-rump length) of the skull diameter or the length of the thighbones. On the basis of this, an estimate of the gestational age and thus the probable end of pregnancy is possible by comparison with statistics. The measurement of the crown-rump length (SSL) in the embryonic period (1st trimester , up to the end of the 12th week of pregnancy) is the most precise method of determining the gestational age The calculated date becomes more and more inaccurate with increasing gestational age (gestational age) but due to individual growth differences.

Consequences of determining the calculated due date

After the calculated date , the start of maternity leave is determined in Germany .


All of the data given below corresponds to information from the Swiss Federal Statistical Office .

At the due date

Distribution of the gestational age of hospital births in Switzerland in 2006.
Birth probabilities of hospital births in Switzerland in 2006 (see text for interpretation).
  • Around 90 percent of all children are born between their 37th week and their 42nd week: they are called “term-born”. About nine percent are born before this period, that is, they are premature , and only about one percent are born after that and are referred to as transferred births . It should be noted that the natural course is almost always influenced here in order to prevent transmission .
  • Only about four percent of births actually occur on the day of EGT.
  • According to the results of a systematic review from 2017, an increased body mass index of the mother is associated with a higher probability of giving birth after the calculated date.
  • The distribution of the time of birth can be seen in the statistics opposite. Of the total of 71,990 births, only those were evaluated for the graphic in which the EGT was plausible and the due date determined by ultrasound did not differ from the EGT by more than a week. There were still 55,905 evaluable dates of birth.
  • The graphic below shows two probability calculations derived from this. Examples for interpreting the values:
    • blue curve: "By EGT (i.e. 40W + 0T) almost 60 percent of all pregnant women have already given birth" "Only 20 percent of all pregnant women give birth before 38W + 2T" Etc.
    • Yellow curve: "For a pregnant woman at the time 40W + 0T there is a nearly 13 percent chance of giving birth on this day. If no birth takes place, the probability increases daily: at the time 42W + 0T the pregnant woman even has a chance of about 55 percent gave birth that day. "

The distribution of the time of birth shows that there is an above-average number of births on the first day of the 41st week (40w + 0T). This and the increased probability on the first day of the 42nd week can be explained by human intervention.

On the day of the week of birth

  • Significantly fewer children are born at the weekend (average number of births per day in 2004: 151) than from Monday to Friday (average number of births per day in 2004: 191). The reason lies in the fact that the time of birth can be influenced by human activity, in particular in the increasing number of planned caesarean sections on weekdays.
  • Wednesday and Friday achieve slightly higher birth rates than the other days of the week. (Switzerland 2004)

At the hour of birth

  • Most often, around eight percent, children are born between eight and nine in the morning. In the case of planned deliveries, the caesarean section that is scheduled is probably the reason for the accumulation.
  • At all other hours of the day, the percentage is four to five percent. Very few children are born between two and three o'clock at night (3.2 percent). (Switzerland 2004)

See also



Individual evidence

  1. Calculating the length of pregnancy based on menstrual data is very often confusing. If the duration of pregnancy is calculated as the time from the first day of the last normal menstruation to the day of birth, it should be taken into account that the first day is day zero and not day one. Days 0 - 6 therefore correspond to "completed pregnancy week 0", days 7-13 to "completed pregnancy week 1" and the 40th actual week of pregnancy corresponds to "completed pregnancy week 39". Archived copy ( memento of the original from March 4, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.dimdi.de
  2. Section 218, Paragraph 1, Sentence 2 of the Criminal Code
  3. AM Jukic, DD Baird, CR Weinberg, DR McConnaughey, AJ Wilcox: Length of human pregnancy and contributors to its natural variation. In: Human reproduction. Volume 28, number 10, October 2013, pp. 2848-2855, doi : 10.1093 / humrep / det297 , PMID 23922246 , PMC 3777570 (free full text).
  4. Sohn, Ch. Et al. Ultrasound in gynecology and obstetrics. 2nd edition, Thieme, Stuttgart 2003, ISBN 3-13-101972-7 , page 66 f
  5. N. Heslehurst, R. Vieira, L. Hayes, L. Crowe, D. Jones, S. Robalino, E. Slack, J. Rankin: Maternal body mass index and post-term birth: a systematic review and meta-analysis . In: Obesity Review . tape 18 , no. 3 , 2017, p. 293-308 , doi : 10.1111 / obr.12489 .
  6. If a caesarean section is planned anyway, it is preferred to take place at 40W + 0T, and at 42W + 0T labor is often induced to prevent transmission .