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How to calculate your gestational age in weeks and months

To know exactly how many weeks along you are and how many months this corresponds to, you need to calculate your gestational age, for which you only need to know the Last Menstruation Date (LMD) and count on a calendar how many weeks there are to the current date.

The doctor can also always determine your correct gestational age, being the date suggested on the ultrasound performed at the prenatal appointment, in order to accurately indicate how many weeks you are pregnant and the estimated date of delivery.

It is also possible to calculate gestational age by entering below the first day of your last menstrual period, to find out how many months you are, how many weeks of pregnancy it corresponds to and on what day the baby is likely to be born:

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What if i don't know the date of my last period?

When a woman is not sure of the date of her last menstruation, the safest way to know the number of weeks of pregnancy and  the probable due date is through an ultrasound or the uterine height, which the doctor usually uses to calculate the weeks and months of pregnancy. For this, the amount of Beta HCG in the blood and the uterine height, after the 12th week of pregnancy are considered, together with an ultrasound requested by the gynecologist.

How to calculate your gestational age in weeks and months

1. How to calculate in weeks

To calculate gestational age in weeks, note the date of your last menstruation on a calendar. Every 7 days, as of this date, will count as another week of life for the baby.

For example, if the first day of the last menstrual period was 11th of March and the result of the pregnancy test is positive, to know the gestational age, one should begin counting the pregnancy from the 1st day of the last menstrual period and not the day the sexual relationship occurred.

If March 11th, which was the LMD, was a Tuesday, the following Monday the baby will have completed 7 days and the gestational age continues increasing every 7 days. If today was April 16th, Wednesday, the baby would have a gestational age of 5 weeks and 2 days, that is, 2 months of pregnancy.

The calculation is done in this way because although you may not yet be pregnant, it is very difficult to define exactly when fertilization occurred, because sperm cells can survive up to 7 days in the woman's body before fertilizing the egg and actually initiating a pregnancy.

2. How to calculate in months

According to the Ministry of Health (2014) determining the gestational age, converting the weeks into months, should be worked out as follows:

1st trimester1 month

up to 4 and a half weeks of pregnancy

1st trimester2 months

4 and a half weeks to 9 weeks

1st trimester3 months

10 to 13 and a half  weeks of pregnancy

2nd trimester4 months

13 and a half  weeks to 18 weeks of pregnancy

2nd trimester5 months

19 to 22 and a half weeks  of pregnancy

2nd trimester6 months

23 to 27 weeks of pregnancy

3rd trimester7 months

28 to 31 and a half weeks of pregnancy

3rd trimester8 months

32 to 36 weeks of pregnancy

3rd trimester9 months

37 to 42 weeks of pregnancy

Usually the pregnancy lasts 40 weeks, but the baby can be born between 39 and 41 weeks without problems. However, if labor does not start spontaneously until the 41-week mark, the doctor may choose to induce labor with oxytocin to the vein.

How to calculate the estimated due date

To calculate the probable due date, which should be around 40 weeks after the LDM, it is necessary to add 7 days to the LDM, then count 3 months back and then add one year.

For example, if the LDM was March 11, 2018, by adding 7 days, the result is March 18, 2018, and then subtract 3 months which means December 18, 2017 and then add 1 year. So, in this case, the probable due date is December 18, 2018.

This calculation does not give the exact due date because the baby can be born between 37 and 42 weeks of pregnancy, however, the mother does know in advance the estimated due date.

Bibliography >

  • PEREIRA, Ana Paula E. et al. Determinação da idade gestacional com base em informações do estudo Nascer no Brasil. Cad. Saúde Pública. Vol 30. 59-70, 2014
  • JYOTHSNA, B. et al. Gestational Age Correlation by Last Menstrual Period, Ultrasonography and Newballardscore. International Journal of Contemporary Medical Research. Vol 5. 7 ed; 77-83, 2018
  • HALL, Eric S. et al. Evaluation of Gestational Age Estimate Method on the Calculation of Preterm Birth Rates. Matern Child Health J. 2013
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