Pregnancy

Correspondence between pregnancy weeks, months and trimesters

The math of pregnancy can get confusing pretty quickly. Are you in week 12 or 13? Your first trimester or your second? Three months or four months pregnant?

When we talk about  pregnancy dating, we generally prefer to consider it in  weeks + days , according to what gynecologists call gestational age . The reason is simple: weeks always have 7 days, while months have a variable number of days, which makes the division between months and weeks  imprecise .

So how to calculate the weeks of pregnancy and their correspondence with months and trimesters?

If you want to know exactly how many weeks you are, you can use our calculator , which will show you the number of weeks and days of gestation , the corresponding month and the days remaining to the expected date of delivery .

Alternatively, you can convert months into weeks of pregnancy or vice versa by consulting the table below . In this way you will be able to know, for example, at how many weeks the third month of pregnancy begins, rather than when the fourth or seventh ends.

Correspondence between weeks and months of pregnancy : table

Printable version

If you wish, you can download a printable version of the conversion table by clicking on the link below.

Calculating pregnancy weeks: why start from scratch?

As you will have seen from the table, pregnancy conventionally begins at 0 weeks + 0 days (0+0) , which corresponds to the first day of the last menstrual period .

This is because the date of conception cannot be known precisely , while that of the cycle is certain.

Correspondence between weeks and months of pregnancy : table

Printable version

If you wish, you can download a printable version of the conversion table by clicking on the link below.

Calculating pregnancy weeks: why start from scratch?

As you will have seen from the table, pregnancy conventionally begins at 0 weeks + 0 days (0+0) , which corresponds to the first day of the last menstrual period .

This is because the date of conception cannot be known precisely , while that of the cycle is certain.

Gestational age: what week am I?

Probably, like many expectant mothers, you may feel confused when it comes to gestational age , which is represented by two numbers separated by a “+” . 

For example, your gynecologist might say you are 11 weeks + 2 days (11+2), which is week 12 of your pregnancy . Why?

11+2 means that 11 full weeks plus 2 days have passed since the first day of your last period. As a result, you are in week 12 , but you haven’t finished it yet. It’s a bit like with age : your child will be in their first year of life until their first birthday and will then be zero years and a certain number of months old.

To better understand the count or for simple convenience, you can consult our pregnancy weeks chart .

How many days and how many weeks does a pregnancy last?

Pregnancy lasts 40 weeks , which is 280 days . Remember, however, that only about 30 percent of expectant mothers give birth after exactly 40 weeks . 

This happens because the expected date of delivery is conventionally calculated, based on the first day of the last menstrual cycle.

At how many weeks is the pregnancy complete?

Based on the guidelines of the Ministry of Health , based on the duration of the pregnancy, we can speak of:

  • full-term pregnancy , if it ends between 37 and 41 weeks;
  • preterm pregnancy (or premature birth) if it ends before 37 weeks;
  • protracted/overterm pregnancy (or postterm birth), if it ends at 42 weeks or more.

How many weeks are there in a quarter?

In a pregnancy, there are 13 or 14 weeks per trimester . In fact, it is not possible to have the same number of weeks for all three quarters, because dividing 40 by 3 the division is not exact.

How many trimesters are there in a pregnancy and how long do they last?

Pregnancy consists of three trimesters : the first trimester , which lasts from approximately week 1 to week 13, the second trimester , which lasts approximately from week 14 to week 27, and the third trimester , which lasts from approximately week 28 to week 40 .

Does pregnancy last nine or ten months?

The 40 weeks of pregnancy are counted as nine months . However, one might wonder why, if there are 4 weeks in a month, there are only 9 months of gestation and not 10.

The explanation is very simple: four weeks correspond to 28 days, while the months (with the exception of February) all have 30 or 31 days . Consequently, for each month, approximately 4.3 weeks should be considered .

Simplified table: trimesters, months and weeks of pregnancy

If these explanations didn’t help, here is a short outline of how to roughly count  the trimesters, months and weeks of gestation:

First quarter

  • 1st month: weeks 0 to 4
  • 2nd month: weeks 5 to 8
  • 3rd month: weeks 9 to 13

Second quarter

  • 4th month: weeks 14 to 17
  • 5th month: weeks 18 to 22
  • 6th month: weeks 23 to 27

Third quarter

  • 7th month: weeks 28 to 31
  • 8th month: weeks 32 to 35
  • 9th month: weeks 36 to 40

Do you want to meet other mothers who will give birth in the same month as you?

There is a  great community  waiting for you! You can find it in our forum, in the “Mamme del…” area dedicated to expectant mothers and divided by month of delivery. Take part too: they are eager to share their experiences with you!

Kathryn Barlow is an OB/GYN doctor, which is the medical specialty that deals with the care of women's reproductive health, including pregnancy and childbirth.

Obstetricians provide care to women during pregnancy, labor, and delivery, while gynecologists focus on the health of the female reproductive system, including the ovaries, uterus, vagina, and breasts. OB/GYN doctors are trained to provide medical and surgical care for a wide range of conditions related to women's reproductive health.

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