How To Calculate Pregnancy Weeks And Months?

6 min read

Written by Editorial Team

Editorial Team

calculate pregnancy weeks and months

Congratulations!!! Your pregnancy test has come positive! One of the first things most women want to know after they realize they are pregnant is to figure out their due date. What month will the baby be born? What would be the zodiac sign? How big is the fetus now?

To answer all these questions, you need first to how pregnancy weeks and months calculate you are. How do you do that? How long does a baby stay inside? Is it 38 weeks? Or 40 weeks? Whoa! You are only pregnant for a few minutes, and you are confused already! Let us break down the whole “pregnancy weeks and months” calculation for you in simple steps.

In This Article

How is My Due Date Calculated?

It might come across as a bit weird. But the first day of your pregnancy is not assumed to be the day you conceived. It is the first day of your last menstrual period (LMP). While it is “technically” not correct, medical practitioners use the LMP for one simple reason – you can rarely say for sure (that is, 100% sure) the day you conceived.

Even if you know the date of intercourse with your partner, did you know that sperm can hang around inside you for up to 3 days before fertilizing your egg? So, the reality is there is no way to say when you conceived, or when ovulation occurred exactly.

But you can say with all surety when you had periods last, provided your periods are regular, and the cycle is 28 days. And this is the reason why you start with your LMP. The week you had your last periods considered as the first week of pregnancy.

What if My Periods Are Irregular?

ultrasound to calculate due date

The due date calculation estimate works best when you have regular cycles, and you get your periods every 28 days. Because in case your periods are irregular, you will not know your LMP and ovulation date cannot be ascertained, arriving on your due date can b difficult.

In such a scenario, your first ultrasound scan or dating scan will give you a viable due date. The sonographer will measure your baby from crown to length, essentially meaning from your baby’s head to bottom. This measurement is a good accurate idea of how far along your pregnancy is.

[Read : When Will I Have My First Scan During Pregnancy?]

How Long Will My Pregnancy Last?

Okay, now you have the starting date. So how many weeks or days from that date do you pregnancy weeks and months? Most pregnancies last 38 weeks to 42 weeks.

But for the sake of convenience and standardization, your due date is calculated 280 days from your LMP. Two hundred eighty days or 40 weeks.

Again, technically, even if you give birth on the exact due date, your baby is not 40 weeks old – this is because you most likely conceived in the middle of your menstrual cycle, that is, around two weeks after your LMP. So your baby stays inside your womb only for 38 weeks. And you are almost always two weeks ahead in your pregnancy.

Essentially, your pregnancy for 40 weeks lasts:

  • 9 calendar months (30 or 31 days each)
  • 10 lunar months (28 days each)

How Are Pregnancy Weeks And Months Calculated?

pregnancy wheel due date calculator

Pregnancy weeks and months calculate in the below manner by the pregnancy week calculator.

In the first week of your pregnancy (based on LMP), you are zero weeks of pregnancy. In the second week, you are one week of pregnancy. That means, in the nth week of your pregnancy, you (n-1) weeks pregnant.

While this might sound a bit confusing, it works exactly the way our age works. When you have celebrated your 30th birthday, the whole of the following year, until your next birthday, you say you are 30, although you are in the 31st year of your existence!

So, when you are in the 10th week of pregnancy, you say you are nine weeks of pregnancy. The same rule applies for “months” as well. In the first month of your pregnancy, you are zero month pregnancy (as silly as that sounds). In the second month, you say you are one month pregnant and so on. You calculate the pregnancy weeks and months in this way.

How Doctors Calculate Pregnancy Weeks and Months?

Doctors calculate pregnancy weeks and months by using a set of three methods to count pregnancy weeks – based on LMP, using ultrasound, and thorough physical examination. The actual time of a delivery cannot be known by this, and the three-step process just helps to predict the date of birth.

1. Physical Examination Method

One can know one’s pregnancy week by the size of the uterus. In the 12 weeks, your medical practitioner can feel the uterus above the pelvis.

After 18 weeks, the distance between the fundus and the pubic bone will be the same as the no of weeks from the menstrual period last. After 20 weeks, the fundus is at the umbilicus. It gives some idea about the pregnancy week. This is not the right way to predict the gestational age always due to factors such as obesity, twin pregnancy, uterine tumors, or the fundus being higher or lower.

[Read : Fundal Height in Pregnancy – How to Measure Pregnant Belly Size?]

2. Using Ultrasound Method

Ultrasound works when you are not aware of the LMP or are irregular periods. It is based on a measurement of the gestational sac and the fetus.

The sonographer calculates the baby from head to toe. It gives the crown-rump length or CRL. It helps the doctor to estimate the pregnancy weeks based on the growth of the baby.

There is not much difference between LMP and ultrasound measurements, and the doctors may use the LMP to establish the due date. For instance, your first-trimester LMP and ultrasound estimate to fall within the same week, LMP considers as the final one. If there is a difference of one or more weeks, the ultrasound estimate consider.

Remember, Your Due Date is an Estimate

So yes, you calculated 40 weeks from the LMP, and ta-da you have the due date. However, please remember that this is only an estimate. Only very few women, to the tune of a meagre 5%, deliver exactly on the due date. The actual date for a full term can be any time between the 38th week to 42nd week, so a week on either side of the due date. Why? Because your baby has not seen your calculation! You plan everything keeping the due date in mind, but always remember it is an estimate!

What Are the Trimesters?

pregnancy trimester

Your pregnancy is divided into three trimesters, each of them lasting about 3 months. Because most pregnancies last for about 9 months, each trimester is about 3 months long and is a standard across the world. The break up of the trimesters is as follows:

  1. First Trimester: Beginning of your pregnancy till about you are 13 weeks and 6 days
  2. Second Trimester: Begins when you are 14 weeks and lasts till 27 weeks plus 6 days
  3. Third Trimester: Starts when you are 28 weeks till the time you have your baby

I hope you have successfully calculated the “age” of your pregnancy weeks and months and the due date. Sure, once you visit your gynecologist, they will give you a rough due date. And once you have your first pregnancy scan, you will get a clear indication of how big your baby is inside you.

And in any case, there are multiple online “due date calculators” available, where you just key in the date of your last menstruation, and you get the entire pregnancy timing charted out. However, there is some charm left in marking your calendar yourself! In this way, you can calculate your pregnancy weeks and months.

Here is wishing you a happy and safe pregnancy! Take care!

Read Also: Stages Of Pregnancy By Week

Editorial Team,

With a rich experience in pregnancy and parenting, our team of experts create insightful, well-curated, and easy-to-read content for our to-be-parents and parents at all stages of parenting.Read more.

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