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    How to Calculate Pregnancy Weeks by Due Date - Use Gestational Age of Baby

    • 4.7   (6 Votes)

    Most of the pregnant women want to know how to calculate pregnancy weeks by due date. The pregnancy weeks are calculated under 3 trimesters. Week 1-13 is one trimester, week 14-26 is second trimester, and week 27-40 is third trimester. Calculation of due date is never perfect. Most of the babies are born between 27 to 40 weeks.

    Let’s Discuss in More Details About Due Date of Pregnancy:

    How many weeks pregnant I am?

    There is no scientific and medical way to examine the accuracy of your pregnancy. Normally, ovulation is the period where we find most pregnancies taking place, however, it is not assured for every woman when they ovulated and conceived. Due to this reason, medical professionals and doctors count your Last Day of Menstrual Period (LMP) as the baseline to count how many weeks in pregnancy you are.

    However, there are some women who prefer to count their pregnancy weeks keeping their last ovulation date as the baseline for pregnancy. Scientifically as well, this method seems to be accurate as it gives a more accurate picture of your due date. Nonetheless, this method to count pregnancy weeks comes handy when you know the exact fertile window of yours. (1) 

    How long does pregnancy last?

    Normally, pregnancy lasts for 40 weeks from the first day of your LMP (Last Menstrual Period). In another method, an average of 38 weeks is counted as the length of pregnancy from the day of conception. (2) (3)

    How to count pregnancy if I don’t know the LMP?

    Normally it is easier to calculate your pregnancy weeks when you have a regular menstrual period and you are aware of your LMP and ovulation by that way. However, there are many women with an irregular menstrual cycle who find it very difficult to count pregnancy weeks accurately without any medical assistance. In this case, you should opt for an ultrasound scan or a dating scan, which can provide you an almost accurate idea about how many weeks in pregnancy you are currently. (4)

    count how many weeks in pregnancy

    However, the disclaimer is that this ultrasound should not be taken as the most accurate calculation of your pregnancy weeks. There may be a variation of a day or more but still works better for women with irregular cycles. Under this scan, the sonographer/doctor measures the crown-rump length (CRL) of your baby which is nothing but measuring the baby from his/her head to bottom. Normally, a dating scan should be done after approximately 10 weeks of your LMP.

    What is Gestational Age?

    If you are counting your pregnancy weeks through your LMP, actually your baby is approximately two weeks younger. Gestational age is nothing but the time length between the first day of your last menstrual period (LMP) to the actual age when the baby is conceived. Therefore when your medical report says you are 6 weeks pregnant, you are actually 4 weeks pregnant. (5) (6)

    What is Trimester?

    A trimester is a method of counting pregnancy through months. Your 38 weeks or 9 months of pregnancy is divided into 3 trimesters where one trimester is equivalent to 3 months. (7)


    How Do I know which Trimester I am?

    The first trimester begins from the date of conception to 13 weeks and 6 days. Similarly, the second trimester is from 14 to 27 weeks and the third from 28 weeks to approximately 38-40 weeks of your LMP. Based on the current week of pregnancy, you can now determine in which trimester you are. (8)

    How accurate is my due date?

    As stated above, no one can tell your accurate due date. Numerically speaking, only 5-10% of babies are born in the actual due date which the doctor/health center provides. Nevertheless, most of the babies are born between 37-42 weeks of the pregnancy. (9) (10)

    Hope, this article has helped to resolve your queries on how many weeks in pregnancy you are, along with other Frequently Asked Questions (FAQs) around this issue. Follow OVO Fertility blog to get regular updates on fertility health and treatments.





    Mushkan Tanwar

    Does breech baby pregnancy bring in some changes in the due date of pregnancy?

    Oct. 3, 2018, 5:30 a.m. 5.0


    Sujata Mishra

    Morning sickness is troubling me a lot. Please tell in which trimester it will get over?

    Sept. 28, 2018, 10:12 a.m. 4.5


    Damini Jha

    Is it safe to have abortion pills after the second week of the first trimester?

    Aug. 24, 2018, 12:53 a.m. 4.5


    Harsita Rawal

    Thanks for sharing! With this article now I can calculate my current week on my own.

    May 21, 2018, 9:11 a.m. 4.5



    I'm 14 weeks pregnant, feeling headache for last week on and off. My blood pressure is normal and so is the baby's heartbeat. I used paracetamol as the doctor recommended me. Is this safe to use 2 tablets twice a day? No other symptoms and I'm worried that headache causes blood pressure issue in th…

    April 2, 2018, 3:24 p.m. 4.5


    Beena Lata

    I always used to call my gyne every time I had an issue with my due date and current pregnancy week. This article is easily understandable and I should have had this during my pregnancy.

    Feb. 12, 2018, 9:58 a.m. 5.0

    View all comments

    Leave a Comment



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    It is not advised to self-diagnose or treat any medical condition or disease following the information given in this article. We insist you to consult a registered practitioner and never try to replace their advice or discontinue treatment in between by relying solely on the information obtained. External links to different websites as well as videos given on the website serve the purpose of sharing knowledge only. Ovo Health being an Independent Medical Information platform is neither responsible nor guarantees the authenticity, reliability, and accuracy of these videos and websites in any way. We do not intend the information displayed here to be used for a medical emergency, if you seek medical attention for yourself (or any other person with whom you want to share the information with), we advise you to directly get in touch with the hospital or the doctor.

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