The estimated date of birth based on the last menstrual period is calculated based on the 40th week of pregnancy , 280 days, 9 months and 10 days. For this purpose, in women with regular menstrual periods, the estimated date of birth based on the first day of the last menstrual period (LTM) is calculated according to a formula called Nagele's rule or law.
In patients with irregular menstrual periods, the gestational week may not be consistent with the last menstrual period. If the last menstrual period is unknown, the date of the first pregnancy test, the date when pregnancy was first suspected, and the date when fetal movements were first felt may provide an idea. However, the estimated date of birth can be calculated based on the estimated last menstrual period corrected by the ultrasonography taken during the examination.
Pregnancy planning is usually not done consciously. Surprise pregnancies usually occur. People who plan their pregnancies consciously do so because they have some special reasons. The most important factor in this is that the baby's due date is desired. In surprise pregnancies, there are some criteria for calculating the baby's estimated due date. However, these are completely estimated. While the birth process is taking place, many factors can affect the date of birth. Calculating the estimated due date will allow expectant mothers to prioritize their preparations during this process and to prepare for birth more consciously.
Estimated date of birth calculation method
The accepted normal pregnancy period covers a period of 40 weeks following the fertilization of the egg in the uterus or in the fertilization channels. This period should be evaluated as 40 weeks from the date of the last menstrual period. In other words, the estimated date of birth is calculated according to the approximate 40th week of pregnancy. However, in reality, births usually occur before 40 weeks. The weeks of birth are between 37 and 40. If there is a risk of premature birth during pregnancy, birth occurs before the estimated date calculated. In some pregnancies, birth can occur in the 41st and 42nd weeks. When the sperm from the man fertilizes the egg cell released from the woman's ovaries, it is considered that pregnancy has begun. However, the day of sexual intercourse during which conception occurs is not taken into account in the calculation of gestational age. In order to provide a standard with this calculation in the world, the first day of the woman's last menstrual period is considered the beginning of pregnancy. The month is not used in the calculation of gestational age. The duration of pregnancy is normally accepted as 280 days in humans. This period is considered to be a period that will continue for 40 weeks in pregnancy. For this reason, when calculating the estimated date of birth, the calculation is made by taking into account the first day of the last menstrual bleeding. Since this calculated date is completely estimated, it only helps the expectant mothers and the doctor.
If the birth does not occur on the estimated date calculated
The estimated date of birth is calculated as 40 weeks according to the expectant mother's last menstrual period. In reality, there may be occasional differences in the date of birth calculated according to the last menstrual period. In this case, if necessary, the corrected date of birth calculated by the obstetrician with the ultrasound measurements is taken into account. For this reason, ultrasound measurement papers are usually used for expectant mothers in the first months of pregnancy. It is necessary to carefully protect the papers or pictures given at the end of this ultrasound. If the expectant mother's pregnancy completes 40 weeks according to both the last menstrual period and the ultrasound result and the birth does not occur, some tests are performed according to this data and if there is any problem according to the results of these, the intervention is made by waiting until the 41st and 42nd weeks at most for the birth.
When is a cesarean section performed if the baby does not arrive on the estimated due date?
If the birth is previously calculated or delayed according to other data, a cesarean section decision should be made and the birth should be performed. However, if a cesarean section is planned or decided in advance, it is usually performed at 39 weeks, that is, one week before the calculated date of birth. If an emergency cesarean section is required, a cesarean section can be performed at any week of the pregnancy. In this case, the estimated date of birth will not be important.
How is the estimated date of birth calculated?
In order to calculate the estimated date of birth in pregnant women, the Nagele method is used. According to this method, 7 days are added to the date when the last menstrual period started and an estimated date of birth is easily calculated by going back 3 months from this date. The estimated date of birth calculated in this way is considered as the latest date that birth can occur. Births usually occur before these calculated dates, that is, between weeks 37 and 40. In some cases, birth may occur on a date 1 week later than the calculated date. These measurements are evaluated in this way only in singleton pregnancies. In the case of multiple pregnancies, births occur before 40 weeks of pregnancy. The period in these births is usually between weeks 34 and 37. Babies born in this way are defined as late preterm.
Example calculation for date of birth
Example 1
Estimated due date for a person whose last menstrual period was June 10
10 + 7 = June 17
Going back 3 months from June, March = March 17 will be the estimated birth date.
Example 2
Estimated date of birth for a person whose last menstrual period was January 5
5 + 7 = January 12
When you go back 3 months from January, October = October 12 will be the estimated birth date.
In order to calculate the estimated date of birth in this way, the expectant mother must have a regular menstrual cycle. In this calculation method, calculations made in women with irregular menstrual periods may cause errors. If such a situation occurs, ultrasound measurements should be used and the estimated date of birth should be calculated in this way. Similarly, it is necessary to take into account the possibility that expectant mothers are not sure of their last menstrual period date, ovulation occurs unexpectedly early or late, and they calculate their menstrual period incorrectly. Since these values are based entirely on estimates and birth is subject to various factors, differences may occur between the actual date of birth. In order not to take risks in these cases, ultrasound examinations performed in the first 3 months of pregnancy should be taken into account and the date that will be closest to the actual date of birth should be determined. In some cases, if the estimated date of birth is calculated incorrectly, effects such as the expectant mother feeling the baby's movements late may be experienced. Due to these factors, all alternatives should be evaluated to ensure a healthy birth.
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