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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 it amounts to you need to calculate the gestational age and to do so you only need to know the Last Menstruation Date (DLM) and count in a calendar how many weeks there are to the current date.

The doctor can also always inform the  correct gestational age, which is the date suggested on the ultrasound performed at the prenatal appointment, to accurately indicate how many weeks you are pregnant and what is the estimated date of delivery.

It is also possible to calculate the gestational age by putting in the first day of your last menstrual period, to know 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 my last menstrual period date?

When the woman is not sure what the last menstrual day was, the safest way to know how many weeks of gestation she has and what the probable due date is through ultrasound or uterine height, which is what the doctor usually uses to calculate the weeks and months of pregnancy. For this, the amount of Beta HCG in the blood, uterine height, which is considered after the 12th week of gestation and ultrasound are requested by the gynecologist.

How to calculate your gestational age in weeks and months

How to calculate gestational age 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 on March 11th, which was the DLM, it was a Tuesday, the following Monday the baby has completed 7 days and it goes on totaling 7 in 7. If today is April 16th, Wednesday, the baby has 5 weeks and 2 days of gestation, which are 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 can survive up to 7 days in the woman's body before fertilizing the egg and actually initiating a pregnancy

How to calculate gestational age in months

According to the Ministry of Health (2014) to know the gestational age, converting the weeks into months, it should be done as follows:

1º Trimester1 monthup to 4 weeks and a half of gestation
1º Trimester2 months4 weeks and a half to 9 weeks
1º Trimester3 months10 to 13 weeks and a half of gestation
2º Trimester4 months13 weeks and a half of gestation to 18 weeks
2º Trimester5 months19 to 22 weeks and a half of gestation
2º Trimester6 months23 to 27 weeks of gestation
3º Trimester7 months28 to 31 weeks and a half of gestation
3º Trimester8 months32 to 36 weeks of gestation
3º Trimester9 months37 to 42 weeks of gestation

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 DLM, it is necessary to add 7 days to the DLM, then count 3 months back and then put the following year.

For example, if the DLM 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 another 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 gestation, however, the mother is already informed of the likely time of the baby's birth.

Baby's growth

During each week of gestation, the baby grows about 1 to 2 cm and gains about 200 g, but in the third trimester it is easier to notice this rapid growth, since the fetus  has all its organs formed and its body starts to concentrate, especially in accumulating fat and preparing for the time of birth.

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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