Calculate your Pregnancy by Months, Weeks and Trimester

An average pregnancy prolongs for around 40 weeks or 280 days. This date is calculated from the first day of your last period. The first day of your last menstrual period (LMP) is calculated as being the first day of your pregnancy, or day one. So even though you probably did not conceive until at least two weeks later, doctors compute the pregnancy from the first day of your last period itself. [1]

Calculating pregnancy weeks to months and then keeping track of the pregnancy from weeks, months, and trimester is a complicated affair. Furthermore, calculating your due date is not an exact science, as very few women end up delivering on their due date. It is nice to have an idea about when your baby is going to be born, but remember not to get too attached to the exact date given by the doctor. [2]

Check on how to calculate your pregnancy weeks to months below.

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How to calculate your due date?

How to Calculate Your Due Date?

The do-it-yourself toothpaste pregnancy test can be done very easily at home. It involves adding a couple of drops of urine to a little bit of toothpaste. The chemical reaction that results will tell you whether you are pregnant or not. You are supposed to use plain white. If you have a regular menstrual cycle of 28 days, then there are two ways of calculating your pregnancy due date. 

1. Naegele's Rule for Calculating Due Date

 The first way of calculating your pregnancy due date is with the Naegele's rule, which involves a simple calculation and is easy to perform.[3] To do this, you add seven days to the first day of your last menstrual period and then subtract three months from that and jump one year ahead to get the due date. [4]

 For example, if the first day of your last known period was on November 1, 2018, adding seven days will mean November 8, 2018. Now subtract three months - August 8, 2018. Now change the year and skip ahead by one year to the year 2019. 

The pregnancy's due date will thus be August 8, 2019.[5]

2. Calculating your Due Date with the Pregnancy Wheel

 Another way of calculating your due date is by using the pregnancy wheel.[6] This method of computing is the most common method used by doctors, and it becomes easy to determine the date when you have access to a pregnancy wheel [7]

The first step to this process is by determining the date of your last period on the pregnancy wheel. Then line up this date with the indicator on the wheel, and it will display your pregnancy due date. Again, remember that having the baby on that exact date is rare, and the due date is only an estimate. 

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How to calculate due date if you don't know your LMP?

What happens if you don't know your LMP?

This is more common than what you might think. However, there are still many ways to figure out your due date, even if you cannot remember the first day of your last period's exact date. 

Here's how to calculate your pregnancy due date if you do not know the date of your last period:

●    If you have an idea around which particular week you had your last period, your doctor can estimate the due date. 

●    If you did not have any idea about when your last period was, then your doctor will prescribe an ultrasound to determine the due date.

See: What To Do After Positive Pregnancy Test

Calculating the Weeks, Months, and Trimester in your Pregnancy

An average pregnancy prolongs for around 40 weeks or 280 days. Only about 30 percent of all pregnancies end up reaching week 40. [8] There are three trimesters in any pregnancy, with each trimester being made up of 13 or 14 weeks. 

There are three trimesters in any pregnancy. The first trimester extends from week 1 to week 13. The second trimester lasts from around week 14 to week 27, and the third trimester lasts from week 28 to week 40 or upwards in some cases.[9]

An average pregnancy of 40 weeks is counted as being nine months. However, since there are approximately four weeks in each month, this means that 40 weeks comes to a total of 10 months. However, four weeks have 28 days, and months, except for February, have 30 or 31 days. This calculation makes each month roughly 4.3 weeks long, which is why pregnancy is calculated to last for nine months.   

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How many weeks are in a full-term pregnancy?

How Many Weeks makes a Full-Term Pregnancy?

For a single baby, full-term pregnancy is considered to be between the beginning of 39 weeks to the end of 40 weeks. However, if you are pregnant with twins, then a full-term pregnancy lasts for around 38 weeks. For more than two babies, the number of weeks keeps decreasing. 

The American College of Obstetricians and Gynecologists (ACOG)  lists the following terms decided for a pregnancy, depending on when the baby is born: [10]

●    Delivery between 20 to 37 weeks: Preterm baby

●    Delivery between 37 weeks 0 days to 38 weeks six days: Early term baby

●    Delivery between 37 weeks 0 days to 40 weeks six days: Full-term baby

●    Delivery between 41 weeks 0 days to 41 weeks six days: Late-term baby

●    Delivery between after 42 weeks 0 days: Post-term baby

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Summary

Even after calculating your due date through any which method, your doctor is still going to order frequent ultrasounds to keep checking on the health of the growing fetus and to understand how the growth indicates the period in your pregnancy. Your doctor will also prescribe you a second due date based on ultrasound measurements. The estimated due date from your LMP and the ultrasound due date will rarely be the same. You must work together with your doctor to estimate the proper delivery date and ensure a healthy pregnancy. 

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References

1. Hunter, L.A., 2009. Issues in pregnancy dating: revisiting the evidence. Journal of midwifery & women's health, 54(3), pp.184-190.

2. Mongelli, M., Wilcox, M., and Gardosi, J., 1996. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. American journal of obstetrics and gynecology, 174(1), pp.278-281.

3. Nguyen, T.H., Larsen, T., Engholm, G., and Møller, H., 1999. Evaluation of ultrasound‐estimated date of delivery in 17 450 spontaneous singleton births: do we need to modify Naegele's rule?. Ultrasound in Obstetrics and Gynecology: The Official Journal of the International Society of Ultrasound in Obstetrics and Gynecology, 14(1), pp.23-28.

4. Loytved, C.A.L., and Fleming, V., 2016. Naegele's rule revisited. Sexual & Reproductive Healthcare, 8, pp.100-101.

5. Darwish, N.A., THABET, SM, NASR, ALA, EL SHARKAWY, SHERIF, and EL TAMAMY, MN, 1994. Modified Naegele's Rule for Determination of the Expected Date of Delivery Irrespective of the Cycle Length. Med. J. Cairo Univ, 62(1), pp.39-47.

6. Chyjek, K., Farag, S., and Chen, K.T., 2015. Rating pregnancy wheel applications using the APPLICATIONS scoring system. Obstetrics & Gynecology, 125(6), pp.1478-1483.

7. Grzybowski, S., Nout, R., and Kirkham, M., 1999. Maternity care calendar wheel. Improved obstetric wheel developed in British Columbia. Canadian Family Physician, 45, p.661.

8. Alexander, J.M., McIntire, D.D., and Leveno, K.J., 2000. Forty weeks and beyond: pregnancy outcomes by week of gestation. Obstetrics & Gynecology, 96(2), pp.291-294.

9. Spencer, K., Bindra, R., and Nicolaides, K.H., 2003. Maternal weight correction of maternal serum PAPP‐A and free β‐hCG MoM when screening for trisomy 21 in the first trimester of pregnancy. Prenatal Diagnosis: Published in Affiliation With the International Society for Prenatal Diagnosis, 23(10), pp.851-855.

10. Acog.org. 2020. Methods For Estimating The Due Date. [online] Available at: [Accessed 20 June 2020].

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