Generality
The calculation of the weeks of pregnancy is an important estimate to take into consideration to know the most probable date of birth and to monitor the development of the fetus inside the uterus.
The duration of gestation is approximately 280 days, ie 40 weeks taking as a reference the beginning of the last menstruation . However, it is difficult for the date of birth to coincide with the one planned. Generally speaking, it can be said that if the menstrual cycles are 28-30 days and the pregnancy proceeds normally, the birth of the child can occur at any time, between two weeks before and two weeks after the scheduled date.
For a better approximation, however, it is possible to take into consideration some individual factors (such as the date of the presumed ovulation or unprotected sexual intercourse), which will then be supported by the gynecological evaluation, laboratory analyzes and ultrasound investigations .
What's this
Pregnancy Weeks Calculation: what does it consist of?
To date the pregnancy, doctors generally refer to the day of the beginning of the last menstruation . The duration of gestation is approximately 280 days, ie 40 weeks starting from the day taken as a reference.
The estimated date of birth, then, is calculated by adding nine months and a week (280 days, in fact) to the first day of the last menstruation.
Pregnancy: why is it calculated in weeks?
First of all, a common conviction must be dispelled: the pregnancy does not last exactly nine calendar months, but ten lunar months, which coincide with 280 days (40 weeks).
The pregnancy dating is carried out in weeks to make precise indications, in consideration of the fact that the weeks are always made of 7 days, while the months have a variable number of days (eg October has 31 days; November 30 and so on ).
Pregnancy Weeks Calculation: When Gestation Begins?
Conception takes place when the spermatozoa, going up the uterus, reach the mature egg cell, released from the ovary during ovulation and available for fertilization.
The fertilized egg is transported by the movement of the hair cells of the tube in the direction of the uterine cavity, where it lurks in the endometrium approximately 6-7 days after unprotected intercourse (ie around the 21st day of a regular menstrual cycle of 28 days, if the fertilization took place on the fourteenth day).
At this point, the production of the chorionic gonadotropin also begins, whose "beta" subunit is important for the functioning of the pregnancy test .
To learn more: When to take a pregnancy test »The period in which it is possible for the egg to be fertilized starts 4-5 days before ovulation and ends 1-2 days later. This is possible in view of the fact that the mature egg cell, when it is expelled from the ovary, manages to survive for about 24 hours, while the spermatozoa can remain viable in the female genital apparatus for up to 72-96 hours. Therefore, unprotected sexual intercourse even 3 or 4 days before ovulation can lead to fertilization.
To know
In technical terms, we talk about:
- GESTATIONAL AGE, when the duration of pregnancy is calculated starting from the first day of the last menstruation. Gestational age is expressed in complete days or complete weeks;
- CONCEPTIONAL AGE, when the count starts from the moment of the presumed conception.
Pregnancy Weeks Calculation: How Long Does Gestation Last?
NORMAL DURATION
The duration of pregnancy is conventionally established in 40 weeks .
As anticipated, this period of time is however approximate, so much so that the parts that took place between the 37th and 41st week of gestation are still normal .
PRE-TERM AND POST-TERM PARTY
When the duration of pregnancy is less than thirty-seven weeks (complete) one speaks of premature birth or pre - term ; similarly, the parts that occur beyond the term, that is, starting from the 42nd week onwards, are considered post-term .
How to do it
Pregnancy Weeks Calculation: How is it done?
By convention, the calculation of the weeks of pregnancy takes as a reference the first day of the last menstruation: whereas a "standard" gestation lasts 280 days, seven days must be added and three months removed from this date ( Naegele rule ).
Example:
- Starting date of the last menstruation : 21 August;
- Expected date of delivery : 28 May.
Clearly, the calculation of the weeks of pregnancy provides an indication of the presumed date of birth. Generally speaking, it can be said that if the menstrual cycles are 28-30 days and the pregnancy proceeds normally, one can expect to give birth at any time, between two weeks before and two weeks after the scheduled day with the count.
Pregnancy Weeks Calculation: What Should Be Considered?
The duration of the pregnancy is calculated in weeks, starting from the day of the beginning of the last menstruation : for this reason, it is important to know this date with certainty.
- Calculation Weeks Pregnancy according to the Menstrual Cycle - The calculation of the weeks of pregnancy thus reported is all the more true, the more regular are the menstrual cycles. A menstrual cycle is considered regular when repeated at 28-day intervals (in short, it should be remembered that this period is calculated by counting the number of days from the first day the menstrual flow appears until the day before the start of the next menstruation). However, a menstrual frequency of 25 to 36 days and a certain individual variability are to be considered normal (the duration of the cycle can change, in fact, from month to month, due to the intervention of different factors).
- Calculation Weeks Pregnancy according to Ovulation - Still assuming a certain regularity of menstrual cycles, the duration of gestation can be calculated starting from the presumed age of ovulation, bearing in mind that this always occurs 14 days before the last flow (luteinic phase) and, on average, 14 days after the beginning of the last menstruation (follicular phase). In the latter case, the variability is greater and depends on the regularity of the cycle: the period preceding ovulation (follicular phase) does not, in fact, have a fixed periodicity and, although it has an average duration of about 14 days, it can undergo oscillations ranging from 1 to 3 weeks; it is precisely from the duration of the follicular phase that the variations in the duration of the menstrual cycle derive. For most women, however, the luteinic phase (from ovulation to the beginning of menstruation) is more constant and takes from 12 to 16 days (average duration: 14 days).
- Calculation Weeks Pregnancy according to the Cohital Report - In addition to the date of the presumed birth, the calculation of the weeks of pregnancy allows to establish, with a certain margin of error calculated in excess or defect, the day of conception. Obviously, the opposite is also valid, that is, one can know with certainty the exact day of fertilization in the event that a single and isolated sexual relationship has been consummated after the last menstruation and that this has been followed by amenorrhea (absence of menstrual flow) .
In summary :
To calculate the pregnancy weeks with a good approximation, you need to know:
- Start date of the last menstruation ;
- Average length of cycles : from 22 to 45 days, considering that the physiological reference interval is equal to 28 days;
- Average length of the luteinic phase (post-ovulatory) : from 9 to 16 days (generally it is equal to 14 days).
The calculation of the weeks of pregnancy starts from the date of the last period . This reference is the only certain in the woman's possession, if we exclude the use of the ovulation test or the monitoring of the basal temperature to know more precisely when conception takes place.
At the beginning of the last menstrual period, the two weeks of the "useful" menstrual cycle for fertilization are added. The following considerations depend on the specific cases: it is calculated, in fact, that if a pregnancy proceeds normally, without complications, it has an average duration of 280 days (ie 266 days from the moment of conception to the time of birth), equal to 40 weeks.
However, a physiological pregnancy can end safely even in slight advance or postponement, between 37 and 42 weeks.
Use the Online Calculator to: Date the Beginning of Pregnancy »Limitations
Generally speaking, the calculation of weeks of pregnancy must be considered an indication . Certain conditions, in fact, involve a variability of time that makes it difficult to estimate the presumed birth date of the child (eg strong irregularities in the cycle, pathological or iatrogenic amenorrhea, etc.).
It should not be forgotten, then, that the calculation of the weeks of pregnancy also depends on the specific characteristics of each mother: very young women, multipara or smokers have an increased risk of having smaller fetuses for gestational age. Therefore, the risk of an incorrect dating of the pregnancy is not excluded.
The probability that the estimation of gestational age is not precise also increases with the use of the contraceptive pill or other hormonal contraceptives in the three months preceding the last menstruation.
In any case, a more precise calculation of the weeks possible is a starting point : this information can be established with a better approximation by means of an ultrasound check and other medical examinations that are performed during gestation.
Pregnancy Weeks Calculation: When Is It Difficult or Impossible?
The calculation of weeks of pregnancy can be quite difficult, as in the case of:
- Irregular cycles ;
- Date of the last uncertain menstruation ;
- Use of assisted reproduction techniques .
Being a function of the duration of the menstrual cycles, women who present strong irregularities may have an idea, but not establish the probable day of conception with extreme certainty. At other times, it is not possible to refer to the normal rules for calculating the weeks of pregnancy, since fertilization occurred during a period of physiological, pathological or iatrogenic amenorrhea .
The calculation of the weeks of pregnancy does not take into consideration, moreover, the variations to the normal progress of the gestation, as can happen, for example, in case of maternal illnesses, particular incidents or problems that can influence the term delivery.
Pregnancy Weeks Calculation: How Is It Corrected By Your Doctor?
In the event that it is too early or too late, the calculation of the weeks of pregnancy can be "adjusted" by the gynecologist.
The dating of the gestation is established with better approximation through the instrumental investigations, such as the ultrasound of the first trimester . In fact, this exam allows the observation of the gestational chamber and the size of the embryo, allowing the gynecologist to confirm or correct the initial estimate, dating the beginning of the pregnancy with a very low margin of error, close to 3 days.
Obstetric Rule: what it is and how it works
The obstetric slide is the tool used by obstetricians and gynecologists to calculate the expected date of birth . This consists of a numbered disc formed by two superimposed wheels, joined in the center by an eyelet, so that they can turn independently of each other. The lower wheel is divided into 12 sections, one for each month of the year, in turn graded with the days of the month; the upper one indicates the 40 weeks of pregnancy divided into the 3 trimesters. Entering the first day of the last menstruation, the doctor confirms the calculation of the weeks, establishes the expected date for the birth and sets the various controls to monitor the gestation.
Dating Ultrasound (First Quarter Ultrasound)
The dating ultrasound is the first to be performed; this exam is one of three free ultrasound checks scheduled by the national health service during pregnancy. To accurately date pregnancy, the cranio-caudal length (CRL) and the biparietal diameter of the fetus are measured.
The ultrasound dating should ideally take place within the tenth week of pregnancy. The earlier the exam is performed, the more accurate the calculation of the gestational week is.
What is it for?
Pregnancy Weeks Calculation: What Is It For?
The calculation of the weeks is a useful indication to know in which period of the pregnancy the pregnant woman and her child are found. This information is part of the data collected in the anamnesis and therefore contributes to monitoring the growth of the fetus, supporting the various ultrasound scans and the check- ups scheduled during the pregnancy. The risk that we run by not dating the gestational age correctly is to consider a child who, instead, is only younger than we thought and vice versa is lagging behind.
Why is it important to calculate time in pregnancy?
Each gestation period corresponds to a specific phase of fetal growth. Therefore, dating the pregnancy correctly is an important reference for:
- Monitor the development of the child inside the uterus;
- Establish the probable date of conception and the expected day of birth ;
- Evaluate fetal growth parameters ;
- Indicate to the future mother the deadlines not to be forgotten, such as the medical checks scheduled during gestation (blood tests, ultrasounds, etc.).
The calculation of the weeks of pregnancy is also useful to indicate the ideal care plan for the future mother: a premature baby could need more care, as well as a gestation that lasts over 42 weeks makes it necessary to induce labor .
