Calculating weeks of pregnancy

A pregnancy lasts about forty weeks, or about 280 days. Until the first ultrasound it is not possible to date the gestation period precisely, however – thanks to a simple calculator and an international convention – it is possible to estimate the presumed date of conception and the presumed date of delivery.

During the 40 weeks of gestation, the fetus (and before that the embryo) grows day after day and the woman’s body changes with it. The following weekly insights can be used to better understand and follow the progression of pregnancy, both from the point of view of the mother and the future newborn.

Pregnancy weeks calendar

first and second week fourteenth week twenty-seventh week
third week fifteenth week twenty-eighth week
fourth week sixteenth week twenty-ninth week
fifth week seventeenth week thirtieth week
sixth week eighteenth week thirty-first week
seventh week nineteenth week thirty-second week
eighth week twentieth week thirty-third week
ninth week twenty-first week thirty-fourth week
tenth week twenty-second week thirty-fifth week
eleventh week twenty-third week thirty-sixth week
twelfth week twenty-fourth week thirty-seventh week
thirteenth week twenty-fifth week thirty-eighth week
twenty-sixth week thirty-ninth week
fortieth week

Calculating weeks of pregnancy

Once you have ascertained that you are pregnant, to calculate what week you are you will need to keep in mind the date of your last menstruation: for the purposes of the calculation, the first day of your last menstrual period is in fact considered the first day of pregnancy. Having identified this day, thanks to our pregnancy calculator, you will be able to find out which week you are currently in and also calculate the presumed date of birth of the baby.

First day of your last menstrual period
Cycle length

As can be understood from the premises, the estimate of the week of pregnancy calculated in this way is all the more approximate the more regular the woman’s menstrual cycle is. In the case of assisted in vitro fertilization, in order to date the onset of pregnancy, 14 days must be subtracted from the date of artificial insemination of the embryo. After that, the calculation takes place as with natural conception.

How to count the weeks of pregnancy

By convention, the counting of the weeks of pregnancy is not done taking into account the date on which the sperm and the egg meet, as some believe, but – even if conception (or fertilization of the egg) takes place about two weeks after – the count starts from the date of the last menstruation: the 1st week coincides with the first day of the last menstruation, ie it starts when you are not pregnant yet. The due date of delivery is then calculated by adding 280 days to the first day of the last menstrual period. This criterion is justified by the fact that it is very difficult to know the precise moment of ovulation and conception and, on the other hand, it implies that during the first two weeks the embryo does not exist at all.

Although in Italy and abroad it is by now a consolidated practice to measure the period of pregnancy according to the gestational method described above, the fetal method is still used in special cases . This method refers to the fetal age (or age): in this case, the duration of the pregnancy is calculated starting from the first day following the presumed date of conception of the child and, therefore, is 14 days earlier than the gestational age.

Although the estimation and calculation methods seen above are commonly used, ultrasound – ie the vision of the gestational chamber and the calculation of the size of the embryo – remains the best tool for dating a pregnancy. Ultrasound for dating is performed at the first checkup, in the 6th -8th week.

Total pregnancy weeks

Although we commonly talk about pregnancy referring to 40 weeks, we must remember that it is never possible for us to establish with certainty the actual date on which the baby will be born: you could give birth even earlier or later.

Babies born before 37 weeks are preterm; full-term those born between the 37th and 42nd week; post-term those beyond. Doctors typically decide to induce labor if labor does not start within 8 to 10 days after full term.

Calculating trimesters and months of pregnancy based on weeks

The duration of pregnancy is expressed in weeks, months and trimesters. We therefore speak of the 40 weeks of pregnancy, the 3 trimesters of pregnancy or the 9 months of pregnancy.

Weeks of pregnancy in trimesters

  1. The first trimester of pregnancy begins on the first and ends on the thirteenth week.
  2. The second trimester begins in the fourteenth week and ends in the twenty-eighth week.
  3. The third trimester begins in the 29th and ends in the 40th week of pregnancy.

Pregnancy weeks into months

  1. The first month of pregnancy runs from the 1st day of the last menstrual period up to 4 weeks + 3 days
  2. The second month of pregnancy ranges from 4 weeks + 4 days to 8 weeks + 5 days
  3. The third month ranges from 8 weeks + 6 days to 13 weeks + 1 day
  4. The fourth month ranges from 13 weeks + 2 days to 17 weeks + 4 days
  5. The fifth month ranges from 17 weeks + 5 days to 21 weeks + 6 days
  6. The sixth month ranges from 22 weeks + 0 days to 26 weeks + 2 days
  7. The seventh month ranges from 26 weeks + 3 days to 30 weeks + 4 days
  8. The eighth month ranges from 30 weeks + 5 days to 35 weeks + 0 days
  9. The ninth month ranges from 35 weeks + 1 day to 40 weeks + 0 days

Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.

She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform,

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