Calculating pregnancy week: let’s see together how to do it.

January 12, 2024

If you are here, it is because you want to understand how calculating pregnancy week works. This kind of calculation can affect the expected day of delivery and, therefore, all the visits you will have to make during the nine months before that happy event.

So let’s see, together, the easiest methods to make the most accurate calculation possible and know what week we are in our gestation. Below we have prepared a simple calculator for you to see what week you are in by entering the date of your last menstrual period. It’s not exactly an obstetrical ruler, but it’s pretty close.

Below the calculator, we will try to understand more about how this calculation is done and how you should communicate with your gynecologist.



How to figure out what week of pregnancy I am in?

The calculation is quite simple: start from the date of your last period and count the past weeks to date. Always keep in mind that week 1 is from day 1 to day 6, so from day 7 onward you will be in week 2.

Why is this important? The math you and the pediatrician will do may be slightly confusing–because the gynecological calculation of the date of pregnancy is a bit peculiar.

The first week will be week 0.

On day 7, it will trigger week 1 — which is the second week.

Each intermediate day is indicated with a “+” after the week number: for example, the third week + 2 days will be indicated with 2+2.

The 20th week plus 5 days will be indicated with 19+5. And so on.

We know, it can be a bit confusing–but we did not invent this way of counting, and anyway, it works.

Since when is the beginning of pregnancy calculated?

As you may have noticed, although it may seem a bit strange, the beginning of pregnancy coincides (at least in medical calculation) with the day of the beginning of the last menstruation.

Since that day – in fact – your body has not been menstruating, so the first month without menstruation is considered the first actual month in which there is an active pregnancy.

True: conception could have occurred on any day between the first day of your menstrual cycle and the day before your period that then never came, but this convention helps you navigate well through all the obligatory stages of pregnancy, from day one to the first trimester, second trimester, and third trimester.

To support these simple calculations, which in any case remain very useful, we have a series of examinations and tests (such as morphological and nuchal translucency) that expectant mothers must undergo to monitor the progress of the fetus’s growth and to make sure of the health status of the incoming baby and her own: in fact, it is essential for both of them that the mother keeps herself in excellent health, interrupting certain habits that can be harmful (alcohol, smoking, and certain foods and drinks) and managing diet and exercise while keeping in mind that there are two lives to go on now and that situations such as gestational diabetes should be avoided.

Calculating pregnancy week for betas?

Betas are precisely one of the first tests that can be done to confirm a pregnancy. There are some good methods to infer a pregnancy, like basal temperature, but betas are the most precise. A must for anyone who has to undergo IVF techniques, this test can be done when a pregnancy is suspected, to confirm a possible home test.

This test measures the amount of Beta hCG (chorionic gonadotropin), a hormone that the body releases in pregnancy, or the presence of certain types of tumors.

This measurement is useful for various reasons, including calculating weeks of pregnancy and finding out if there is an extrauterine pregnancy.

There are values to guide you and figure out if everything is going well, but especially in the first weeks the ranges are so wide that it may be almost impossible to tell with ease if things are “going” or not. Remember, assessments like this can only be made by your gynecologist.

To understand, this is a general chart that broadly indicates the weeks of gestation based on the amounts and growth of hormone amounts in the blood tested.

  • Negative result: < 5 mIU/ml
  • 1-2 weeks: ≃ 5-500 mIU/ml
  • 2-3 weeks: ≃ 100-5,000 mIU/ml
  • 3-4 weeks: ≃ 500-10,000 mIU/ml
  • Weeks 4-5: ≃ 1,000-50,000 mIU/ml
  • Weeks 5-6: ≃ 10,000-100,000
  • 6-8 weeks: ≃ 15,000-200,000 mIU/ml
  • 8-12 weeks: ≃ 10,000-100,000 mIU/ml

What does the belly look like at 6 weeks?

It is very likely that you are just starting your pregnancy (be careful about thinking it is “just a little”…time flies!) and so you have probably confirmed your pregnancy with a test and maybe even a beta check, but you still don’t see anything on your body.

Remember that the growth of your fetus will be exponential: for a long time it will be a tiny little thing and then, all of a sudden, it will be a continuous doubling, until the ninth month … and beyond (but by then it will be outside the belly). A growth that is at first almost horizontal and imperceptible (without an ultrasound) and then practically vertical.

In the first few weeks, then, apart from a few symptoms that you may or may not feel, your body will not undergo any major visible changes. Maybe you will be able, around 6-7 weeks, to notice a little bulge, or you will get to 12 weeks and barely your body will be transforming and gaining weight. Women are different, we can’t tell you that.

Certainly, in the first period, hardly anyone but you will suspect anything.

But whether you see your belly or not, one thing is certain: it is a perfect time to prepare for all that is to come. Because while the baby isn’t there yet, you have the time and space to do it, and it’s the best thing you can do for yourself, for the next family, and for the baby on the way.

When do you enter month 2 of pregnancy?

As with the counts before, remember to be careful: the first month runs from the first day of your last period until 4 weeks + 3 days (4+3 for the gynecologist). Therefore, it may be best to continue to orient yourself with the week count. The second month, then, will start at 4 weeks and 4 days (4+4) — we told you it can be confusing!

You’ll also notice that when you’re toward the end of your pregnancy, in the seventh and eighth months, people will tend to tell you “You’re almost there!” forgetting that the ninth month has to be completed and you’re hopefully heading to a natural childbirth.

In any case, try to have a shared method with your gynecologist, because that will be how you will orient yourselves together for all the important visits and to see if everything is going well.

What not to do during the first weeks of pregnancy?

The first few weeks are the most critical. By “first weeks,” we mean the first 12. Keep in mind that almost a quarter of pregnancies spontaneously terminate in the first 12 weeks. This is 25%, which is quite a high figure.

That is why it is important to do three things:

  • Take the utmost care of your body and the fetus with the best possible habits
  • Take these first 12 weeks with a fairly light heart, because it can happen to lose it physiologically, through no fault of your own
  • Do not communicate the pregnancy publicly, so as not to risk having to also communicate the premature loss caused by a possible miscarriage

During this period it may also be useful to refrain from intense exercise, to help the implantation of the embryo, but as always the usual rule applies: these assessments can only be made by your gynecologist.

Calculating pregnancy week: the best possible pregnancy

Whatever your timing, whether your pregnancy has just started or has been going on for a while, the best thing you can do is to be as prepared as possible and make sure you have people around you who know and can support you: motherhood is a wonderful adventure, difficult and exhausting while delivering all the joys you deserve.

At Parentalife we want to support you along the whole adventure of pregnancy and parenting and we do it through our courses and consultancies. Feel free to contact us if you feel that you need it!

SOURCES

Edwards KI, Itzhak P. Estimated Date of Delivery. [Updated 2022 Oct 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK536986/

About the Author

Severino Cirillo

Health, Wellness and Education Expert. With a degree in Community Health, he is the CEO of Informed Parent and is responsible for validating the blog's scientific information and coordinating the editorial team of experts, consisting of gynecologists, midwives, psychotherapists, and others in pregnancy, perinatal, and parenting wellness and health.