How Do You Calculate the Weeks, Months and Trimesters of a Pregnancy?
- A pregnancy lasts an average of 266 days, or 38 weeks, calculated from conception.
- However, since the date of conception is usually unknown, your first week of pregnancy officially starts on the first day of your last menstrual period (LMP). This makes a pregnancy approximately 280 days or 40 weeks in total.
- Counting the weeks of pregnancy can be daunting since it is calculated in weeks, months, and trimesters. We’ll break it all down and explain how it works! 😊
Try:
- 🤰 "Am I Pregnant?" — Online Pregnancy Test
- 🗓 Pregnancy Week Calculator – Find out how far along you are!
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Understanding Pregnancy Weeks, Months, and Trimesters
Pregnancies are calculated in weeks, months, and trimesters. The following chart provides an overview of the counting methods:
Trimester | Month of Pregnancy | Week of Pregnancy LMP |
First Trimester | 1 | 1 - 4 |
| 2 | 5 - 8 |
| 3 | 9 - 12 | |
| Second Trimester | 4 | 13 - 16 |
| 5 | 17 - 20 | |
| 6 | 21 - 24 | |
| Third Trimester | 7 | 25 - 28 |
| 8 | 29 - 32 | |
| 9 | 33 - 36 | |
| 10 | 37 - 40 |
Calculating Cut-Off Dates for an Abortion
Perhaps you are experiencing an unintended pregnancy and are faced with the decision of whether or not to abort.
While researching, you might have come across cut-off points of 6 or 22 weeks for a legal abortion. Sometimes the time limit of 63 days is given. It can be difficult to find your way through this confusion, especially when your thoughts and emotions are already tense and disoriented.
- ⏳Try: Until When Can You Get an Abortion? - Find out the legal cut-off point for an abortion in your area.
How many months pregnant am I?
The months of pregnancy are divided into weeks and are calculated according to the lunar calendar. Here, each month consists of exactly four weeks (28 days).
The first day of your last menstrual period (LMP) initiates the beginning of the first month; the fifth week is the beginning of the second month, etc. Therefore, the nine calendar months of pregnancy equal ten lunar months.
- Example: In which month of pregnancy are you at 17 weeks? Divide the 17 weeks by 4 weeks per month. The result is 4.25. This means that at 17 weeks, you are at the beginning of your fifth month.
How do I know how many weeks pregnant I am?
In addition to calculating in weeks, months, and trimesters, there is a more precise method of counting weeks and days.
Example: The last day of the 14th week can also be called 13 + 6.
You calculate it this way: One week consists of seven days. The first day of the 14th week is 13+0, and the last day is 13+6. The next day is 14+0, which marks the beginning of the 15th week of pregnancy.
NOTE: The terms last menstrual period (LMP), gestational age (GA), and menstrual age (MA) can be used interchangeably when determining the length of pregnancy. In most countries, doctors and lawmakers count the weeks of pregnancy from the last menstrual period rather than using the conceptional age, which counts from fertilization.
How do I calculate my due date?
- Starting from the first day of your last menstrual period, you add 9 months and 7 days.
- If you know the exact day of intercourse that led to conception, you can count back 7 days, subtract three months, and add one year.
Remember that the due date is always an estimate, give or take eight days.
Pregnant and Anxious—Where Do I Go From Here?
Maybe you are at the very beginning of your pregnancy and are worried about the weeks ahead. You might still be trying to discern a way forward for you and the baby. Or perhaps you would like to know how much time you have left to make a decision if you are considering an abortion.
- ⏳Until When Can I Get an Abortion? Find out your regional cut-off dates for abortion.
Your questions and concerns matter! Allow us to come alongside you as you discern which path is best for you.
- 🧑⚕️ (Suspecting a) pregnancy — How soon to see a doctor? Quiz
- 🍷🚬 Tobacco/alcohol/drugs: Has my consumption hurt the baby? — Tox Test
- ⛑ Unintended pregnancy? — Quick Answers Quiz
- ⚖️ Should I get an abortion - Abortion Test
You may also like:
- Until When Can You Get an Abortion?
- How Do I Make a Good Decision?
- The Four Most Common Reasons for an Abortion
Sources:
- Moore, K. L. et al. (2024) The Developing Human: Clinically Oriented Embryology, 12th edition, Elsevier
- Kiworr, Michael (2016): Neun Monate bis zur Geburt (Nine Months until Birth), Aachen: Bernardus-Verlag.
- Höfer, Silvia/Scholz, Alenka (2020): Tag für Tag durch meine Schwangerschaft (My Pregnancy Day-by-Day), München: Gräfe und Unzer.
- Schwangerschaftsdauer (Duration of Pregnancy): AMBOS GmbH (26.03.2024)