As we know, these days pregnancy is counted in weeks, but it wasn’t always like this. In the past, pregnancies were counted in months, not weeks1, and this can make things a bit confusing for women. Which month of pregnancy am I in? Pregnancy weeks can indeed be counted as months, you just need to follow the logic. Pregnancy normally lasts around 240 days, counted from the first day of menstruation. But do you know why it’s like this and how to convert weeks into months?

Why the Change from Months to Gestational Weeks?

Because not all bodies are exactly the same, and depending on the woman’s cycle, the fertile period can change2. Some women ovulate on the fourteenth day while others ovulate on the eighteenth day of their cycle, and in these cases it’s impossible to know exactly when conception happened.

That’s why doctors studied and decided to standardize pregnancy tracking in weeks, so you’d know exactly what stage the pregnancy was in at that moment. With advances in ultrasonography, it also became easier to identify the fetus by the number of weeks, because just giving the number of months in the ultrasound report would be too vague.

Imagine a report simply says: pregnant, three months. Yes, but at what point exactly? At the end or beginning of the third month? It’s a period of rapid changes, and this information makes all the difference; these details can be confirmed through a morphological ultrasound. It’s odd to start counting when you’re not actually pregnant yet, and counting in months is much closer to reality for calculating the due date of delivery. Once you get used to counting pregnancy by weeks rather than months, it’s much easier to understand. Since it’s hard to explain to someone who had their baby back when pregnancies were counted by months and not by weeks, I’ve made this table to illustrate and help show your mother, aunt, or grandmother.

Pregnancy Months and Pregnancy Weeks in Months

Months Weeks
1st month 1 to 4 weeks
2nd month 5 to 8 weeks
3rd month 9 to 12 weeks
4th month 13 to 16 weeks
5th month 17 to 21 weeks
6th month 22 to 26 weeks
7th month 27 to 30 weeks
8th month 31 to 35 weeks
9th month 36 to 40 weeks

Division of Weeks

Normally, these weeks are grouped by trimester, so they’re broken down as follows:

  • 1st to 12th weeks = 1st trimester
  • 13th to 26th weeks = 2nd trimester
  • 27th to 40th weeks = 3rd trimester

When the LMP Calculation Doesn’t Match the Ultrasound

And what if the ultrasound doesn’t match the calculation by the doctor using the last menstrual period? It’s simple: the ultrasound actually determines what stage the pregnancy is in. Therefore, the pregnancy weeks from the initial ultrasound report are what should be considered.

It’s important to know the date of your last period to determine which day of the week your pregnancy week is completed. For example: if your period started on a Monday the 5th, you’ll probably complete weeks every Monday. It may seem complicated, but talk to your doctor—they’ll guide you so there’s no doubt about figuring out your probable due date3.

Hope this helps! Kisses

Important Tip: Supplementation during pregnancy is essential, especially in the early weeks. That’s why Famivita has developed an excellent vitamin supplement that already contains Methylfolate (the active form of folic acid). It’s a complete vitamin at a fair price. You can find out more about FamiGesta here.

See also: Fetal development week by week  and the Pregnancy calculator