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General Practice 2023

How to calculate gestational age (in weeks and months)

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How to calculate gestational age (in weeks and months)
How to calculate gestational age (in weeks and months)

To know exactly how many weeks pregnant you are and how many months that means, it is necessary to calculate the gestational age and for that it is enough to know the Date of the Last Menstruation (LMP) and count in a calendar how many weeks there are until the date current.

The doctor can also always inform the corrected gestational age, which is the date suggested in the ultrasound performed at the prenatal consultation, to indicate exactly how many weeks the woman is pregnant and what the Probable Date of Delivery will be.

It is also possible to calculate the gestational age by simply indicating the first day of the last period, to know how many months you are, how many weeks pregnant this means and on what day the baby is likely to be born:

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

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

Learn about other tests that are performed during pregnancy.

How to calculate gestational age in weeks

To calculate the gestational age in weeks, note the date of the last menstrual period on a calendar. Every 7 days from this date, the baby will have one more week to live.

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

This way, if March 11th, which was the DUM, was a Tuesday, on the following Monday, 7 days are completed and adding 7 in 7, if today is April 16th, Wednesday, the baby is 5 weeks and 2 days pregnant, which is 2 months of pregnancy.

The calculation is done like this because although the woman is not yet pregnant, it is very difficult to define exactly when fertilization occurred, as sperm can survive up to 7 days in the woman's body before fertilizing the egg and actually starting the pregnancy.

How to know the gestational age in months

According to the Ministry of He alth (2014) to know the gestational age, converting weeks into months, it should be noted:

1st Quarter 1 month up to 4 and a half weeks of pregnancy
1st Quarter 2 months 4 and a half weeks to 9 weeks
1st Quarter 3 months 10 to 13 1/2 weeks pregnant
2nd Quarter 4 months 13 and a half weeks gestation to 18 weeks
2nd Quarter 5 months 19 to 22 and a half weeks pregnant
2nd Quarter 6 months 23 to 27 weeks gestation
3rd Quarter 7 months 28 to 31 and a half weeks pregnant
3rd Quarter 8 months 32 to 36 weeks gestation
3rd Quarter 9 months 37 to 42 weeks 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 by 41 weeks, the doctor may choose to induce labor with oxytocin in the vein.

See also how pregnancy is week by week.

How to calculate the probable date of birth of the baby

To calculate the probable date of delivery, which should be around 40 weeks after the DUM, it is necessary to add 7 days to the DUM, then count back 3 months and then enter the following year.

For example, if the DUM was on March 11, 2018, when adding 7 days, the result is March 18, 2018, and then it decreases 3 months which means December 18, 2017 and adds plus 1 year. Therefore, in this case, the Expected Delivery Date is December 18, 2018.

This calculation does not give the exact date of the baby's birth because the baby can be born between 37 and 42 weeks of gestation, however, the mother is already informed of the probable time of the baby's birth.

Baby growth

During each week of pregnancy, the baby grows about 1 to 2 cm and gains approximately 200 g, but in the third trimester it is easier to notice this rapid growth, as the fetus already has its organs formed and its body starts to focus mainly on accumulating fat and preparing for the moment of birth.

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