How to calculate the weeks of pregnancy?

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Marie-Ange Demory
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Calculation of the weeks of pregnancy

Pregnancy does not last 9 calendar months, as we usually think, but 10 lunar monthsie 40 weeks from the beginning of the last menstruation to the most probable date of delivery; a physiological pregnancy can be concluded safely between three weeks before and two after that date (therefore between 37 and 42 weeks) and this variability of times leads to the need to know, at every moment of pregnancy, what period and week you are in. How to make a calculation weeks of pregnancy? There are some ways to know what week you are in.






Calculation table of weeks of pregnancy

First month from the 1st day of the last menstruation up to 4 weeks + 3 days
Second Month from 4 weeks + 4 days to 8 weeks + 5 days
Third month from 8 weeks + 6 days to 13 weeks + 1 day
fourth month from 13 weeks + 2 days to 17 weeks + 4 days
Fifth Month from 17 weeks + 5 days to 21 weeks + 6 days
Sixth Month from 22 weeks + 0 days to 26 weeks + 2 days
Seventh month from 26 weeks + 3 days to 30 weeks + 4 days
Eighth month from 30 weeks + 5 days to 35 weeks + 0 days
Ninth month from 35 weeks + 1 day to 40 weeks + 0 days

There is also a limit of another two weeks, by which a pregnancy can reach 42 settimane, which is taken into account when one has to be patient again if labor does not start.



1 month of pregnancy, when does it start?

The first month of pregnancy starts from day 0 and ends at 4 weeks plus three days. The first month of pregnancy begins from conception which occurs approximately 14 days after the last menstruation.



2 month of pregnancy, when does it start?

The second month of pregnancy begins at 4 weeks + 4 days and ends at 8 weeks + 5 days

3 month of pregnancy, when does it start?

The third month of pregnancy begins at weeks and 6 days and ends at 13 weeks and 1 day.

4 month of pregnancy, when does it start?

The fourth month of pregnancy begins at 13 weeks and 2 days and ends at 17 weeks and 4 days. The second trimester of pregnancy has begun.

5 month of pregnancy, when does it start?

The fifth month of pregnancy begins at 17 weeks and 5 days and ends at 21 weeks and 6 days.

6 month of pregnancy, when does it start?

The sixth month of pregnancy begins at 22 weeks and ends at 26 weeks and 2 days.

7 month of pregnancy, when does it start?

The seventh month of pregnancy starts at 26 weeks and 3 days and ends at 30 weeks and 4 days.

8 month of pregnancy, when does it start?

The eighth month of pregnancy begins at 30 weeks and 5 days and ends at exactly 35 weeks.

9 month of pregnancy, when does it start?

The ninth month of pregnancy begins at 35 weeks +1 and ends at 40 weeks.

Weeks of pregnancy calculation obstetric rule

Il Obstetric Rule, amicably called "Ruzzola or Rotella", is a tool used by midwives and health personnel to calculate, during pregnancy, the estimated date of birth and the exact location of specific weeks of pregnancy within the months of the year, for the booking and execution of exams, ultrasounds and blood tests.

It is important to remember that, as they are printed, they are not 100% correct but may differ by 1-2 days from each other. It is now common to find the Obstetric Rule in the kit of documents that accompany the expectant mother: many shops specializing in maternity or children's items often give them as gifts to expectant mothers.

The obstetric slide rule is composed of two discs, one smaller and one larger, made of cardboard or plastic superimposed in the center: the outer disk shows all the days of the year, divided by month (29 February of leap years is not considered, a noteworthy element, for the midwife, if a pregnancy includes that date), while the internal one reports, declined, the weeks of pregnancy and, often, some notes related to them (beginning of the perception of fetal movements, period of increased tendency to abortion or increased risk, periods in which to carry out correctly the tests foreseen for pregnancy).

Using the ruler is possible calculate the weeks of pregnancy starting from the date of the last menstruation and then risking the day of conception. Holding the larger disc still, the internal one is rotated until the first arrow, which reads "Date of last menstruation", is on the day of the start of the last menstrual cycle. Once the arrow has been positioned on that date, keeping both discs still in the position obtained, you will have a clear vision of the location of the different periods and weeks of pregnancy within the solar year, so as to be facilitated in the booking of visits and exams.

There are also numerous Apps or sites that allow you to calculate how many weeks of pregnancy you are at and also give precise and useful information on the development of the fetus and the mother's health status. 

Online pregnancy weeks calculation

On the Internet we find many tools and calculators that allow us to have a complete chart of the weeks of pregnancy. Typically it is sufficient to enter the date of the last menstruation.


Calculation of weeks of pregnancy and months correspondence

Therefore, if we commonly believe that a pregnancy lasts nine months, in reality what is most relevant both in clinical and practical practice is the duration of pregnancy expressed in weeks. This is because the duration of months it is not always the same while a week is always made up of seven days. It is on the basis of these, in fact, that doctors establish thegestational period and the estimated date of delivery.

Let's recap:

  • pregnancy lasts, on average, 40 weeks;
  • you start counting the first week starting from first day of the last menstruation, therefore generally two weeks before the actual conception (which occurs in the fertile period during the days of ovulation), also because it is never certain when fertilization really took place;
  • on the basis of the weeks, the gynecologist establishes the gestational period and the presumed date of birth, also using the obstetric slide;
  • even if during the various ultrasounds the gestational age of the fetus could vary, because of course the baby grows based on when it was actually conceived and not based on the presumed calculation of the weeks starting from the last menstruation, what matters and is taken into consideration, even for a eventual scheduled cesarean delivery, is the progression of the weeks of pregnancy, for which, for example, a scheduled cesarean will be fixed no earlier than the 38th, 39th week of pregnancy;
  • in week pregnancy calculation not only the weeks are considered, but also the days.

Beta Hcg analysis, after how long can they be done?

Le beta Hcg can be done about a week after sexual intercourse during which conception took place, but it is still advisable to wait at least the delay of the cycle to take the home pregnancy test which, in fact, analyzes the presence of these hormones. On average, the test can be done 10-12 days after conception.

Calculating weeks of male or female pregnancy

Naturally, the curiosity to know whether the child to be born is male or female arises immediately, as soon as the positive result on the pregnancy test is read. Even if the baby's sex has been established since conception - depending on whether the sperm transmits the Y or X chromosome - know that there is no way to know if the baby is male or female until the 22nd week, when you will do the morphological ultrasound or when you will do theamniocentesis (in that case the result will be 100% reliable). There are, however, some popular and traditional beliefs that it is possible to know in advance the sex of the unborn child and one of them is the Chinese calendar.

An ancient table recovered in China reveals that it is possible to know in advance the sex of the child by crossing the data relating to the mother's age with the month of conception. And so we find that if we want a girl we should try to get pregnant in April when we are 21, 22 or 29 years old, while if we want a boy the highest odds would be for women of 18,20, 30, 42 and XNUMX years who conceive. their son in July.

Calculating weeks of pregnancy from conception

We have seen that generally, if you have a regular cycle, the conception occurs two weeks after the last menstruation, close to ovulation, but since it is not possible to know the exact day on which conception occurred, the pregnancy is dated from the last menstruation. Of course in some cases, however, we can know when the conception took place and therefore we can have a more precise idea of ​​the age of the fetus. The fact remains that the gynecologist will still use dating from the last menstruation.

Pregnancy week by week

Pregnancy therefore lasts on average 40 settimane, because gynecologists calculate the duration of gestation starting from the date of last menstruation (since you can never be sure exactly when conception occurred). This means that the first two weeks are those that precede conception, which probably occurred around the middle of the cycle, and that the actual pregnancy starts from the third / fourth week.

Video: How to calculate weeks of pregnancy

How are weeks of pregnancy calculated and why is it said that a pregnancy does not last exactly nine months? Why is it important to date the weeks of pregnancy and how does the obstetric rule work and what is it for? At how many weeks can the sex of the baby be discovered?

What month of pregnancy are you?

Do you want to follow your pregnancy step by step from the first month to the last? Here is an in-depth analysis on the whole process of a woman's gestation: what happens month by month, how the body of the future mother changes, how the baby grows, exams to do, what to eat, and many other useful tips

  • First month of pregnancy
  • Second month 
  • Third month 
  • Fourth month of pregnancy
  • Fifth month
  • Sixth month
  • Seventh month of pregnancy
  • Eighth month 
  • Ninth month

Calculation of pregnancy weeks and delivery date

As we have seen, the calculation of the weeks of pregnancy is made starting from the date of last menstruation. This rule is used to date the pregnancy because there is no certainty of the exact date of conception, which generally occurs around the fourteenth day after menstruation.

The estimated date of delivery is after 40 weeks (280 days after the date of the last menstruation).

IVF pregnancy weeks calculation

To calculate the weeks of pregnancy with medically assisted procreation treatments, it is necessary to evaluate both the date of insemination of the oocyte and that of the last menstrual cycle. Regarding the assisted fertilization in vitro it is necessary subtract 14 daysi to the date of artificial insemination of the embryo and thus the onset of pregnancy can be dated. After that the calculation takes place as for natural conception. 

Questions and answers

How are the months of pregnancy calculated?

We generally think that a pregnancy lasts nine months but in reality gynecologists measure it in weeks, a certainly more precise method, because each week lasts exactly 7 days while the months have a variable duration. In any case, there are different schemes on the net that allow you to understand which week you are in and in which week you pass from one month to another in your pregnancy. The first trimester is from 0 to 13 weeks; the second trimester is from 14 to 28 weeks, the third trimester is from 29 to 40 weeks.

How are weeks of pregnancy counted?

The weeks of pregnancy, which are 40 in all, start counting from the first day of the last menstruation even if conception probably occurred two weeks after the cycle. So even if the pregnancy actually lasts 38 weeks, in the clinical setting there are 40.

How to know how many weeks it is?

To find out how many weeks of gestation we are at, just use a ruler, a simple obstetric instrument, or one of the tools found on the net: just enter the date of the last menstruation to get the estimated date of birth and at what week of pregnancy there is is situated.

26 weeks how many months are they?

At 26 weeks and 3 days we enter the seventh month of pregnancy. So the twenty-six weeks is a watershed between the sixth and seventh month of pregnancy.


Audio Video How to calculate the weeks of pregnancy?
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