How To Calculate Pregnancy Weeks And Months Accurately?
If you have conceived recently and are pondering how to calculate your due date, you may need a week-by-week pregnancy calculator.
You may be already doing the calculations to figure out the due date. By using different calculation methods, you may end up with different dates. However, doctors have standard methods of calculating these dates. Moreover, the due date is determined differently for women who have irregular periods or have conceived through in vitro fertilization.
This post will tell you in detail about how to calculate weeks and months during pregnancy.
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. The time between your menstrual period and ovulation is 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 after conception and not 40 (1).
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, the 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?
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 a delivery cannot be known, the three-step process helps you predict the likely date of birth.
1. Physical examination:
One can know one’s pregnancy weeks 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 (4).
2. Ultrasound method:
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.
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 (3).
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 how many more weeks you have to reach a full-term pregnancy.
PREGNANCY DUE DATE CALCULATOR
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., if they 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 a 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 six 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 deliver 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. Forty weeks adds up to nine calendar months and a week (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 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 it is during natural conception. This is because the exact date of embryo transfer is known. The insemination or egg retrieval procedure 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 of 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.
It is for this reason that when you miss your first period, you are four weeks pregnant even though you are in your fifth week of pregnancy (i.e., starting of a new month).
How Accurate Is Your Pregnancy Due Date Calculation?
Due dates are just estimates, which differ from one pregnancy to another. According to a British study, only 4% are known to be born on their due date, and 90% are born before or after the due date (12). Most babies arrive in between 37 and 41 weeks of pregnancy.
Frequently Asked Questions
1. Can my due date change?
Yes, it is normal for your due date to change depending on the growth of your baby, which may be detected in the subsequent ultrasounds (13).
2. Why is it important to know my exact pregnancy due date?
Knowing your pregnancy due date is essential to schedule tests and doctor’s appointments and monitor fetal growth according to your gestational age (14).
3. What is Naegele’s rule for due date calculation?
Naegele’s rule for due date calculation comprises three steps based on the 28-day menstrual period. The steps are (15):
- Determining the first day of the last menstrual cycle
- Counting three calendar months preceding that date
- Adding one year and seven days to that date
Pregnancy calculator week by week may help you know the exact gestational age to plan diet, activities, and medical appointments recommended for the specific week. This may also help you plan prenatal exercises according to the pregnancy age. You may calculate weeks of pregnancy from the last menstrual period since it can be closer to the ovulation and fertility window. Those who undergo assisted reproductive procedures or track ovulation may have a more accurate gestational age. You may seek healthcare providers’ help to calculate the due date and age of the pregnancy.
2. S Rowlands and P Royston; Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?
3.Pregnancy and birth: When your baby’s due date has passed; Institute for Quality and Efficiency in Health Care (IQWiG); Cologne, Germany
4. Estimating gestational age; Medical Education Division at the Brookside Associates
5. Amita A. Mahendru, et al.; Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing
6. Estimating Gestational Age from Fundal Height Measurement; Openlearn; The Open University
7. S Rowlands and P Royston; Estimated date of delivery from last menstrual period and ultrasound scan: which is more accurate?
8. Multiple Pregnancy: The Management of Twin and Triplet Pregnancies in the Antenatal Period.; National Collaborating Centre for Women’s and Children’s Health (UK).
9. Maternal Health And Nutrition; The Missouri Department of Health and Senior Services
10. M. Jukic, et al.; Length of human pregnancy and contributors to its natural variation
11. Wennerholm UB, et al.; Gestational age in pregnancies after in vitro fertilization: comparison between ultrasound measurement and actual age. NCBI
12. Keith Moore; How accurate are ‘due dates’?
13. Due date calculato; Cleveland Clinic
14. Methods of Estimating the due date; ACOG
15. Calculating a due date; Johns Hopkins Medicine
16. Pregnancy dating; NCBI
17. Estimated Date of Delivery; NCBI