How To Calculate Your Conception and Baby Due Dates

If you’ve been trying to conceive, a positive pregnancy test result would bring a lot of joy, excitement, expectations, and questions. In fact, one of the top questions on your mind may be how to calculate your conception and due dates.

Don’t fret: we’re here to help you.

In this article, you’d learn the process of calculating your conception and due dates, and also find answers to every question you have about these dates.

Keep reading to find out.

First, What Is Conception?

In plain terms, conception is the fertilization of a mature egg by a mature male sperm. This unique process occurs as a fertile sperm cell moves up through your vagina and into your uterus.

During ovulation, your body releases an egg from your ovaries. As this egg travels to your uterus, the sperm cell joins and eventually fertilizes it.

Conception date: sperm hitting the egg

After fertilization, the egg continuously divides as it moves down the fallopian tube. In about a week, the fertilized egg would have traveled to the uterus and become a growing mass of living cells called a blastocyst.

Eventually, this blastocyst attaches to and subsequently becomes embedded into the uterus through a process called implantation. If this process is successful, it causes your body to release certain pregnancy hormones which stop menstruation and promote the growth of your little one.

A culmination of all these specific processes is what is commonly referred to as conception.

How To Calculate Your Conception Date

For women with regular 28-day periods, conception usually occurs between the 11th and 14th day after the first day of her Last Menstrual Period (LMP).

Experts agree that pregnancy lasts for about 40 weeks and it begins from the first day of your last menstrual period (LMP).

The estimation of your conception date is usually based on this. However, it is difficult to be 100% certain because it is not easy to know exactly when ovulation occurred. Furthermore, this process is even more tricky for women who have irregular periods or do not remember the first day of their last period.

Notepad for baby due date and conception date calculation

If this is the case, we recommend getting an ultrasound scan done for a more accurate estimation of gestational age or conception date.

Nevertheless, these calculations aren’t fixed. Only a few women get to deliver on their due date. Therefore, we advise that you consider the date as a possible range and not a fixed fact. Considering Jerky Spastic Movements During Pregnancy can also be handy

How Can I Calculate My Baby Due Date?

If your periods follow a regular 28-day cycle, you can calculate your due date using any of these two methods:

1. Pregnancy Wheel

This is the method your doctor is most likely to use.

In fact, it is very easy to calculate your due date if you have access to a good pregnancy wheel.

A pregnancy wheel, or gestation calculator, is a small calendar that uses your LMP to calculate your due date.

Pregnancy wheel in how to calculate baby due date

The first thing to do is locate the date of your LMP on the wheel, then line that date up with the indicator and the wheel would display your due date.

However, it is always important to remember that the due date is just an estimate of when you will deliver your little one. It is actually not set in stone, and the chances of actually going into labor or giving birth on that exact date are very slim.

2. Naegele’s Rule

This method involves a simple calculation.

Here’s the stuff: Add 7 days to the first day of your LMP and then subtract three months.

For example, if your LMP was July 1, 2021:

  • Add 7 days ( July 8, 2021)
  • Subtract three months (April 8, 2021)
  • Change the year (if necessary)

Due Date: April 8th, 2022

What If I Can’t Remember The Date of My Last Menstrual Period (LMP)?

Don’t worry. This is actually more common than you think.

Thankfully, you can still calculate your due date even if you can’t remember the first day of your LMP.

All you have to do is speak with your doctor and tell him/her the week you had your last menstrual period. With this info, your doctor can easily estimate your due date.

However, if you don’t know when you had your last period, your doctor can still determine your due date by conducting an ultrasound scan.

Can My Baby Due Date Change?

Yes, your doctor may change your date if your little one is smaller or larger than the average size at a particular stage of pregnancy.

In most cases, your doctor will conduct or order an ultrasound to determine the gestational age of your baby if your LMP is uncertain, there’s a history of irregular periods, or if you got pregnant even though you weren’t trying to conceive.

This ultrasound scan would enable your doctor to measure the crown-rump length (CRL) – the length from one end to the other of your little one. In the first trimester, this measurement provides the most accurate estimation of gestational age.

Ultrasound check for baby health

Your doctor may change your due date based on the CRL measurement gotten from an ultrasound.

Why Do I Have Two Due Dates?

It’s not uncommon to have two estimated due dates.

After an ultrasound, your doctor will write a report based on his/her observations and include two possible due dates:

  • The first is calculated using your LMP date
  • The second is based on the measurements gotten from the ultrasound

Most times, these dates are different.


Furthermore, if you have more ultrasound scans, each report will have a new due date based on the most recent measurement gotten. However, measurements taken in the second or third trimester will not cause a change in your expected due date.

In Conclusion

Conception and due dates are more accurate in the early days of pregnancy. In addition, it is helpful to remember that these dates are just estimates and things may eventually follow a different timeline. Thankfully, ultrasounds provide more accurate information on how your little one is growing and when he/she would come to join you in this world.

Enjoy the journey, Mama.