Calculating pregnancy due date week by week

How To Calculate Pregnancy Weeks And Months Accurately?

September 14, 2017

When do you give birth to a baby? Is it after the completion of nine months, or 40 weeks; are both the same or different? If you have just tested positive for pregnancy, you might be doing those calculations already, trying to figure out the due date for your delivery. Don’t be surprised if you are coming up with a different date every time you calculate.

MomJunction understands the difficulty in narrowing down on the exact weeks of pregnancy and thereby the due date. Therefore, we will tell you how to calculate pregnancy weeks and months . Read on!

How Is Pregnancy Due Date Calculated?

Your pregnancy due date is calculated by adding 40 weeks (i.e., 280 days) to the first day of your last menstrual period (LMP). This is done with an assumption that you have a 28-day menstrual cycle. Menstrual period and ovulation period are calculated as the first two weeks of pregnancy as ovulation takes place two weeks after the first day of LMP. If you deliver the baby on the estimated due date, she is 38 weeks old, and not 40 (1).

If you miss your first period, you are likely to be four weeks pregnant (including two weeks of menstruation and ovulation) (2).

How To Calculate Weeks Of Pregnancy?

Pregnancy is calculated based on your last menstrual period.Week one of your pregnancy will start on the first day of your LMP. Likewise the following weeks are calculated. This may sound weird but it helps in giving the exact date for timely medical care. Unless you had an assisted conception and knew when the fertilization took place, LMP method is the only way to know the closest ovulation and conception dates (3).

The doctors can help you with the exact calculation of weeks and months of pregnancy through other ways.

How Do Doctors Date Pregnancy Weeks?

The doctors and midwives use a standard set of three methods to count pregnancy weeks – based on LMP, using ultrasound, and through physical examination. Though the actual timing of delivery cannot be known, the three-step process helps you predict the likely date of birth.

1. Physical examination:

You can know your pregnancy week by the size of the uterus. Around 12 weeks, your medical practitioner can feel the uterus (fundus) above the pelvis. After 18 weeks, the distance (in cm) between the fundus and pubic bone will be the same as the number of weeks from the last menstrual period. In about 20 weeks, the fundus is at the umbilicus. This gives a rough idea about the pregnancy week. However, this is not always the right way to predict the gestational age due to factors such as twin pregnancy, obesity, uterine tumors, or the fundus being higher or lower (5).

2. Ultrasound dating:

Ultrasound works even when you are not aware of the LMP or if your periods are irregular. It is based on a series of measurements of the gestational sac and the fetus.

The sonographer measures the baby from head to toe. This gives the crown rump length or CRL. It helps the doctor estimate the pregnancy week based on the growth of your baby (6).

If there is not much difference between ultrasound measurements and LMP, the doctors may use the LMP to establish the due date. For instance, if your first-trimester ultrasound and LMP estimates fall within the same week, LMP is considered final. If there is a difference of one week or more, the ultrasound estimate is considered (4).

Try our pregnancy due date calculator to know your due date. It gives you the estimated conception date and estimated due date along with the weeks of each trimester. It will also show your current pregnancy week, and many more weeks you have to reach a full-term pregnancy.

Pregnancy Due Date Calculator

When Do You Start Counting Pregnancy Weeks?

You can start counting pregnancy weeks as soon as you calculate your due date based on your LMP. You will be counting your 40 weeks from the first day of LMP. As you near the due date, you may have to start counting your pregnancy by days as – 39 weeks one day, 39 weeks two days, and so on (6).

What If Your LMP is Not Known Or Periods Are Irregular?

The due date calculation works ideally if your periods are regular, i.e., follow a 28-day cycle.

If your periods are irregular or you do not remember your last period, you should wait for your first ultrasound pregnancy scan. Also known as dating scan, it will provide the number of weeks remaining for your due date (7).

If the scan does not give the correct results, your doctor may ask you to wait for some more time. Ideally, the dating scan will be done between ten and 13 weeks plus six days (8).

What Are The Three Trimesters In Pregnancy?

The pregnancy period is divided into three, three-month segments, known as trimesters. The division helps understand the changes in the body.

First trimester: It lasts from week one to week 13, when you experience nausea and tiredness.

Second trimester: It lasts from week 14 to week 28. You will feel much better in this period and your pregnancy bump will be seen. Early pregnancy symptoms such as morning sickness, headache, dizziness, fatigue, etc., slowly disappear.

Third trimester: It lasts from week 29 until you birth the baby. You will require extra energy to help your baby grow and beat the tiredness (9).

Does Pregnancy Really Last For Nine Months?

It is based on calculation. 40 weeks adds up to nine calendar months (with 30 or 31 days each) or ten lunar months (with 28 days each) (10).

For instance, if your last period was on January 1, your estimated due date would be October 7 or 8. It equals to nine months and one week (and can be longer if you go beyond the due date).

How Do You Calculate Pregnancy Weeks After IVF (In Vitro Fertilization)?

Calculating weeks of pregnancy and the due date is more precise during an IVF than natural conception. This is because the exact date of embryo transfer or ovulation is known. The insemination or egg retrieval method is scheduled before ovulation in IVF, thus making it more accurate.

The IVF due date is calculated by adding 38 weeks (i.e., 266 days) to the day the eggs were inseminated.

An alternative calculation is by adding 38 weeks to the date of transfer, and subtract the age of embryos from this date. For instance, if it is a three-day embryo transfer, subtract three days and if it is a four-day embryo transfer, subtract four days (11).

It, therefore, gives reliable estimated due dates. Also, with progressing pregnancy, the estimated due date could be more accurate based on ultrasound scans.

Week-to-Month Pregnancy Chart:

If these calculations are making you go nuts, there is a simple breakdown on how the pregnancy weeks, months, and trimesters line up with each other.

If You Are In Your Fifth Week Of Pregnancy, Why Are You Referred To As Four Weeks Pregnant?

Sarah Owen, author of “The Complete Guide To Pregnancy And Child Care”, helps clear the confusion about pregnancy weeks by relating them to a baby’s age. When a baby is one year old, she completes the first 12 months of her life. Though she enters the second year, she is called one-year-old. Similarly, when you complete the first week of pregnancy, you are known to be one-week pregnant even though you enter the second week.

A quick look to understand the terms and counting of pregnancy weeks:

  • In your first week of pregnancy, you will be referred to as being zero weeks pregnant
  • In your second week of pregnancy, you will be referred to as being one week pregnant.
  • In your third week of pregnancy, you will be referred to as being two weeks pregnant.
  • And so on.

How Does A Pregnancy Calculator Work?

You are pregnant. Congratulations! Your doctor might have informed you with the help of a pregnancy calculator. Now, you might be eager to know your due date. Well, However, some of you might be curious how they come to know when exactly your bundle of joy is about to arrive! So, let’s take a look at various methods to calculate your due date.

Last Menstrual Period (LMP) Method

This is the standard method. You add 280 days to day one of your last menstruation. This method works on the classic 266-day pregnancy period plus 14 days. The latter is the time of the first day of menstruation till you would have ovulated. However, this pregnancy calculator might be erroneous; as the assumption is that you have ovulated 14 days after the commencement of your menstrual period and were fertilized exactly on that day.

Basal Body Temperature And Lutenizing Hormone (LH)

This is yet another pregnancy calculator. You keep a check on your basal body temperature or the surge of LH. It indicates the occurrence of ovulation, the time of intercourse, and the date of fertilization. If you are sure of your fertilization date, then you simply have to add 266 days to get your due date.

Naegle’s Rule

This is a faster way to know your due date. Many doctors follow this pregnancy calculator. It takes into account the first day of your last period along with the standard gestation time. If you know this date, simply count three months back from your date and add 7 days to this. You will get your due date.


In an ultrasound test, sound waves are used to produce your baby’s picture. When you are in the first trimester, your doctor may use ultrasound test to estimate your baby’s age. On the basis of this, its due date can be calculated.

Remember; there’s no accurate method of calculating the date of delivery. Mostly, they are calculated on the basis of averages; hence, they are hardly precise. It has been assumed by medical experts that, on an average, a baby remains in the womb for about 40 weeks. However, pregnancies may last a week or two earlier or later than this.

To get into some statistics, it has been found that about 80 per cent of mothers deliver within 10 days of their delivery date while 10 per cent of them deliver in less than 38 weeks. There are about 8 per cent babies who are born after 42 weeks. Quite a long stay in the womb!

Well, whatever may be the due date; forget worrying about the exactness of pregnancy calculator. Concentrate on enjoying each moment of your pregnancy. Good Luck!

Pregnancy Calculator: How to Calculate Pregnancy Due Date Week By Week

Stages in Pregnancy Until 8 Months Pregnant

The estimated due date (EDD or EDC) is the date that the spontaneous onset of labor is expected to happen. The due date may be evaluated by adding 280 days (9 months and 7 days) to the first day of the last menstrual period (LMP). This is the method used by “pregnancy wheels”.

The accuracy of the EDD derived from this technique relies on upon exact recall by the mother, expect a regular 28 days cycle, and that conception happens on day 14 of the cycle. Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be answerable to an error of more than 2 weeks.

Pregnancy Due Date and Gestational Age Calculator

The PDD is a tool mothers use to calculate the conception date (date of ovulation, egg retrieval, or insemination). It is a useful used by millions of expectant mother like you around the world.

The tool is also useful in date of 3-day embryo transfer, date of 5-day embryo transfer, the due date by sonogram (reverse calculation), and the first day of last menstrual period. In order to use this tool, visit perinatoloy and make a selection from the list as seen below, select the month, date, year and click calculate to get your results.

As you can see, the PPD on the screen below gives you the full results including the details of your next doctor’s visit and the required screening before the baby’s due date.

In cases where the date of conception is known accurately, for example, such as in vitro treatment, the EDD is calculated by adding 266 days to the date of conception.

Ultrasound uses the size of the focus to determine the gestational age (the time elapsed since the first day of the last menstrual period). The accuracy of the ultrasound calculation of the gestational age varies as indicated by the gestational age.

“Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate system to establish or affirm gestational age”

The American College of Obstetricians and Gynecologists recommends that ultrasound-established dates should take preference over menstrual dates when the discrepancy between ultrasound dating and LMP is

  • Greater than 5 days before 9 0/7 weeks of gestation
  • Greater than 7 days from 9 0/7 weeks to 15 6/7 weeks
  • Greater than 10 days from 16 0/7 weeks to 21 6/7 weeks
  • Greater than 14 days from 22 0/7 weeks to 27 6/7 weeks
  • Greater than 21 days after 28 0/7 weeks.

“Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; they need to be guided by careful consideration of the entire clinical picture and may require closer surveillance, including repeat ultrasonography to ensure appropriate interval growth.”

In Case Pregnancy Calculator Indicates Twins

At the point twin pregnancy is the result of in vitro fertilization determination of gestational age should be produced from the date of embryo transfer. Else, date the pregnancy using the larger embryo.

Different Methods For Estimating the Gestational Age during Pregnancy

Clinical Examination

A pelvic examination supported by good menstrual records in the first trimester has been reported to be a dependable system for dating of pregnancy.

Doppler Ultrasonography

The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients [10]. The gestational age should, therefore, be no less than 10 to 12 weeks if the fetal heart tones are heard.

Human Chorionic Gonadotropin Pregnancy Test

Human chorionic gonadotropin first becomes detectable in the mother’s blood and urine around 6 and 14 days after fertilizations (3 to 4 weeks gestational age) [11-13]. The gestational age would, thus, be at least 3 to 4 weeks at the time of a liable hCG pregnancy test.

Pregnancy Due Date Calculator

Esitemates Due Date is :

Pregnancy due date calculator


Pregnancy is a term, used to define the period of a woman carrying one or two babies until the time of delivery. Child birth, occur usually after 40 weeks of last mensuration or 38 weeks after insemination.

At your first visit to the gynecologist, the doctor tends to tell the approximate date of childbirth. The due date is usually depicted by sonography measurement, but the genuine pregnancy length depends on numerous factors. Such as, the pregnancy of first child has a long-time period compared to following pregnancies after it. Less than ten percent of child births happen on the due date while fifty percent of births occur within a week of given date and ninety percent occur within two weeks of the due date.

In case, you do not know the due date, you can know it by using Pregnancy due date calculator to get an estimated due date.

The total time period of pregnancy can be divided into three sets each, consisting three months, which is called a Trimester. You can know if you are pregnant or not by pregnancy tests which are based on detection of the hormonal level. Urine tests or blood tests can detect pregnancy after 8 days after the fertilization, however, urine tests can detect after fifteen days after the fertilization. Blood tests give an accurate result.

A nutritious and balanced diet is essential for baby and the conceiving mother.

A balanced and nutritious diet is very important for both the mother and baby. An ample amount of periconceptional folic acid should be consumed by the mother as it restricts fetal neural tube defects, which aids in preventing spina bifida. The essential fatty acid, DHA omega-3 should be consumed by the mother as it’s essential in building of retina and brain.

How to use the pregnancy due date calculator?

Using this calculator is simple, you just have to enter the accurate date of last mensuration period, mentioning the days, month and year.

The test will show these following things:

Estimated due date

Ending date First Trimester

Ending date Second Trimester

Estimated Due window to

Estimated Due window from

Estimated Conception Date

Why is pregnancy due date calculator useful?

It’s essential to remember months close to delivery are essential to remember. There are some things that you should either do during the time period or avoid. The date which the calculator provides with gives a better understanding of when the second trimester ends and you have to be cautious about certain things. There are certain things you should avoid during the third trimester, with this calculator you get an idea when the third-trimester starts and you avoid these prohibited acts.

Another reason is that if you have an idea of your due date you will also have an idea of when you’ll have to be admitted to the hospital since mothers are instructed to be admitted two weeks before the due date.

Things to do and avoid in your first trimester.

As described above, ample folic acid is essential especially during the first trimester.

Searching and choosing a health care provider:

It won’t be stressful for you if you have found a doctor or midwife you feel comfortable with, but as for those who haven’t, they should do research. Search online or take suggestions from friends or family.

Smoking can cause problems even it may lead to miscarriage. Furthermore, smoking also inhibits fetal growth and increases chances of death and stillbirth. Avoiding smoking gives chances of a healthy baby.

Stop consumption of alcohol:

Drinking alcohol, even in small amount can increase chances of low birth weight and impact on your child’s speech, learning and hyperactivity.

Reduce caffeine consumption:

High intake of caffeine results in miscarriage and other problems during pregnancy.

Stock pantry with healthy ingredients:

Fill your pantry with pregnancy friendly ingredients like yogurt, fresh fruits, dry fruits, multi grain pasta and nuts.

How to avoid morning sickness:

One-third women suffer from morning sickness during the first trimester. Try consuming small portions of room temperature and bland food and use ginger. If this doesn’t help then with the perception of the doctor consume anti-nausea medication or vitamin B6.

Things to do and avoid in your second trimester.

This may not help to get rid of stretch marks but prevent itchiness.

It’s highly advised that women should visit dentist and take care of oral hygiene since women with good oral hygiene tend to transmit less oral bacteria to their child.

Stay away from sports or unsafe activities

Stay away from activities like surfing, downhill skiing, scuba diving and amusement park rides as they have a risk of falling which is harmful to both mother and baby.

Avoid sleeping on your back

Sleeping on the back is harmful as uterus presses spinal cord along with other major blood vessels which result in back pain. The advised way of sleeping is on left side.

Avoid unnecessary medications

It’s essential to know if the medicine is safe to use during pregnancy as the medicine in the blood stream of a baby through the placenta.

Things to do and avoid in your third trimester.

Mentally prepare yourself for breastfeed

In the third trimester, the gynecologist asks you to prepare your nipples for breast feed. You can do this by massaging the nipples, this also helps in avoiding soreness.

Spas are harmful as foot massages stimulate contraction and foetal activity.

Start shortlisting baby’s name

Most of the friends and family will come up with cute name suggestions. Start on planning what to name your baby.

Sitting for long periods make the mother tired quick and increase in backpain. Standing creates space for baby.

Its essential that you consume 10 8-ounce glasses of water per day. Every woman has different experience and needs during pregnancy, so its nothing to worry about if you require more or less glass of water. Always keep a check on your urine, if the color is dark yellow or if the urine is cloudy you should consume more water.

Sources of information:

Pregnancy Calculator: How to Calculate Pregnancy Due Date Week By Week

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