Pregnancy Week Calculator: How to Estimate the Delivery Date?

Pregnancy Week Calculator: How to Estimate the Delivery Date?


Il calculation of the weeks of pregnancy is an important estimate to take into consideration to know the date more likely of childbirth and for to monitor the development of the fetus inside the uterus.

La duration of gestation is approximately of 280 days, ie 40 weeks taking as reference the beginning of the last menstruation. However, the date of birth is unlikely to coincide precisely with the expected one. In principle, 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 e two weeks after the expected date.

For a better approximation, however, it is possible to consider some individual factors (such as the date of the alleged ovulation or unprotected sexual intercourse), which will then be supported by the gynecological evaluation, from laboratory analysis and by ultrasound investigations.


Pregnancy Weeks Calculator: what does it consist of?

To date the pregnancy, doctors usually refer to the start day of the last menstruation. The duration of gestation is approximately 280 days, ie 40 settimane starting from the day taken as a reference.

The estimated date of delivery, therefore, is calculated by adding nine months and a week (280 days, in fact) on the first day of the last menstruation.

Pregnancy: why is it calculated in weeks?

First of all, a common belief must be debunked: pregnancy does not last exactly nine calendar months, but ten lunar months, which coincide with 280 days (40 weeks).

The dating of pregnancy is carried out in weeks to make the indications precise, in consideration of the fact that the weeks are always made of 7 days, while the months have a variable number of days (e.g. October has 31 days; November 30 and so on ).

Pregnancy Weeks Calculator: When Does Gestation Begin?

Il conception it occurs 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 carried by the movement of the hair cells of the tuba towards the uterine cavity, where it nestles in the endometrium approximately 6-7 days after unprotected sexual intercourse (therefore around the 21st day of a regular 28-day menstrual cycle, if fertilization took place on the fourteenth day).

At this point, the production of the chorionic gonadotropin, whose "beta" subunit is important for the functioning of the pregnancy test.

Learn more: When to Take the 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 after. This is possible in consideration of the fact that the mature egg cell, when expelled from the ovary, can survive for about 24 hours, while the spermatozoa can remain viable in the female genital system for up to 72-96 hours. Therefore, unprotected intercourse even 3 or 4 days before ovulation can lead to fertilization.

Pregnancy Weeks Calculator: How Long Does Gestation Last?


The duration of pregnancy is conventionally established in 40 settimane.

As anticipated, this time frame is however approximate, so much so that the parties that took place between the thirty-seventh e la forty-first week of gestation however fall within the normality.


When the duration of pregnancy is less than thirty-seven (complete) weeks it is referred to delivery premature o pre-dates; similarly, births that occur beyond the deadline, i.e. from the forty-second week onwards, are considered post appointments.

How to do it

Calculating Pregnancy Weeks: How Do You Do It?

By convention, the calculation of the weeks of pregnancy takes as reference the first day of the last menstruation: considering that a "standard" gestation lasts 280 days, it is necessary to add seven days and subtract three months from this date (Naegele's rule).


  • Start date of the last menstruation: August 21;
  • Estimated date of delivery: May 28th.

Clearly, the calculation of the weeks of pregnancy gives an indication of the presumed date of delivery. In general, it can be said that if your menstrual cycles are 28-30 days and the pregnancy proceeds normally, you can expect to give birth at any time, between two weeks before and two weeks after the expected day with the count.

Pregnancy Weeks Calculator: What Should Be Considered?

The duration of pregnancy is calculated in weeks, starting from day ofstart of the last menstruation: for this reason, it is important to know this date with certainty.

  • Pregnancy Weeks Calculation as a Function of Menstrual Cycle - The calculation of the weeks of pregnancy thus reported is all the more truthful, the more regular the menstrual cycles are. A menstrual cycle is considered regular when it is repeated at intervals of 28 days (in short, it should be remembered that this period is calculated by counting the number of days that go from the first day on which the menstrual flow appears until the day before the start of the period. next menstruation). However, a menstrual frequency from 25 to 36 days and a certain individual variability is to be considered normal (the duration of the cycle can change, in fact, from month to month, due to the intervention of various factors).
  • Calculation of Pregnancy Weeks as a function of Ovulation - Always assuming a certain regularity of menstrual cycles, the duration of gestation can be calculated starting from the presumed ovulation period, bearing in mind that this always occurs 14 days before the last flow (luteal 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 presenting an average duration of about 14 days, it can undergo fluctuations, ranging from 1 to 3 weeks; it is precisely from the duration of the follicular phase that any variations in the duration of the menstrual cycle derive. For most women, however, the luteal phase (period from ovulation to the beginning of menstruation) is more constant and takes 12 to 16 days (average duration: 14 days).
  • Calculation of Pregnancy Weeks as a function of Coital relationship - In addition to the date of the presumed birth, the calculation of the weeks of pregnancy makes it possible to establish, with a certain margin of error calculated in excess or in defect, the day of conception. Obviously, the opposite is also valid, that is, it is possible to know with certainty the exact day of fertilization in the event that a single and isolated sexual intercourse has been consumed after the last menstruation and that this has been followed by amenorrhea (absence of menstrual flow) .

To recap:

To calculate the weeks of pregnancy 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 luteal (post-ovulatory) phase: from 9 to 16 days (generally it is equal to 14 days).

The calculation of the weeks of pregnancy starts from date of last menstruation. This reference is the only certain in the woman's possession, if one excludes recourse to the ovulation test or basal temperature monitoring to know more precisely when conception takes place.

To learn more: Ovulation Test - What is it and How does it work?

The two weeks of the menstrual cycle "useful" for fertilization are added to the start date of the last menstruation. 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 (i.e. 266 days from the moment of conception to that of delivery), equal to 40 weeks.

However, a physiological pregnancy can safely end even slightly earlier or later, between 37 and 42 weeks.

Use the Online Calculator to: Dating the Beginning of Pregnancy

Future resarches

Generally speaking, the calculation of the weeks of pregnancy should be considered a'indication. Some conditions, in fact, involve one timing variability such as to make it difficult to estimate the child's presumed date of birth (eg severe cycle irregularities, 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: the very young, multiparous or smokers have an increased risk of having smaller-for-gestational-age fetuses. Therefore, the risk of incorrect dating of pregnancy is not excluded.

The likelihood that the gestational age estimate is inaccurate also increases with the use of birth control pill or other hormonal contraceptives in the three months preceding the last menstruation.

In any case, the most accurate week calculation possible is a start point: this information can be established with a better approximation by means of a ultrasound check and other medical tests that are performed during gestation.

Calculating Pregnancy Weeks: When Is It Difficult or Impossible?

Calculating the weeks of pregnancy can be quite difficult, as in the case of:

  • Irregular cycles;
  • Uncertain date of last menstruation;
  • Recourse to assisted fertilization techniques.

Being in function of the duration of menstrual cycles, women presenting strong irregularities they may have an idea, but cannot determine with extreme certainty the probable day of conception. At other times, it is not possible to refer to the normal rules for calculating the weeks of pregnancy, as fertilization occurred during a period of physiological, pathological or iatrogenic amenorrhea.

Furthermore, the calculation of the weeks of pregnancy does not take into account the changes to the normal course of gestation, as can happen, for example, in the case of maternal illnesses, particular accidents or problems that can influence full term birth.

Pregnancy Week Calculator: How is it corrected by the doctor?

If 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 instrumental investigations, such as thefirst trimester ultrasound. This examination allows, in fact, the observation of the gestational chamber and the size of the embryo, allowing the gynecologist to confirm or correct the initial estimate, dating the onset of pregnancy with a very low margin of error, close to 3 days.

Dating Ultrasound (First Trimester Ultrasound)

L'dating ultrasound it is the first to be performed; this exam is one of the three free ultrasound checks provided by the national health service in pregnancy. To correctly date the pregnancy the cranio-caudal length (CRL) and the biparietal diameter of the fetus.

Ultrasound dating should ideally take place within the XNUMXth week of pregnancy. The earlier the examination is performed, the higher the accuracy of the gestational week calculation.

What is

Pregnancy Week Calculator: what is it for?

The calculation of the weeks is a useful indication to know in which period of pregnancy the pregnant woman and her baby are. This information falls under the data collected in the anamnesis and therefore contributes to to monitor the growth of the fetus, supporting the various ultrasounds and checkups expected during pregnancy. The risk of not dating the gestational period correctly is that of considering a child who is only younger than expected and vice versa as a child who is delayed in growth.

Why is it important to calculate the times during pregnancy?

Each gestation period corresponds to a specific growth phase of the fetus. Therefore, dating pregnancy correctly is an important reference for:

  • Monitor the development of the child inside the uterus;
  • Establish the date probable of conception and the day scheduled for delivery;
  • Evaluate fetal growth parameters;
  • Indicate to the expectant mother the deadlines not to be forgotten, like the medical checks provided during gestation (blood tests, ultrasound scans, etc.).

The calculation of the weeks of pregnancy is also useful to indicate the ideal care plan for the expectant mother: a premature baby may need more care, as a gestation that lasts beyond 42 weeks requires a 'induction of labor.

Continue reading: Beta hCG: What is it? Values ​​and Meaning in Pregnancy
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