How to Calculate Gestational Age Based on Months Weeks?

To know exactly how many weeks of pregnancy you are and how many months this means, you need to calculate the gestational age and for this, you only need to know the Last Menstruation Date (DUM) and count in a calendar how many weeks there are until the current date.

The doctor can also always inform the corrected gestational age, which is the date suggested on the ultrasound performed at the prenatal appointment, to accurately indicate 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 indicating only the first day of the last menstrual period, to know how many months you are, how many weeks of pregnancy this means and on which day the baby is likely to be born:

What if I do not know the date of last menstruation?

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

How to calculate the gestational age in weeks?

To calculate the gestational age in weeks, the date of the last menstruation should be noted on a calendar. Every 7 days, as of this date, the baby will have another week of life.

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

Thus, if on March 11, which was the DUM, it was a Tuesday, the next Monday the following is completed 7 days and totaling 7 in 7 if today is April 16, Wednesday, the baby is with 5 weeks and 2 days of gestation, which are 2 months pregnant.

The calculation is made so because although the woman is not yet pregnant, it is very difficult to define exactly when fertilization occurred because the sperm can survive up to 7 days in the woman’s body before fertilizing the egg and initiate the pregnancy.

How to know gestational age in months

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

1st Quarter1 monthup to 4 and a half weeks’ gestation
1st Quarter2 months4 weeks and a half to 9 weeks
1st Quarter3 months10 to 13 weeks and a half of gestation
2nd QuarterFour months13 and a half weeks of gestation at 18 weeks
2nd QuarterFive months19 to 22 weeks and a half of gestation
2nd Quarter6 months23 to 27 weeks of gestation
3rd Quarter7 months28 to 31 weeks and a half of gestation
3rd Quarter8 months32 to 36 weeks of gestation
3rd Quarter9 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 age of 41 weeks, the doctor may choose to induce labor with oxytocin in the vein.

Also, see how the pregnancy is week by week.

How to calculate the likely date of the baby’s birth?

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

For example, if the DUM was March 11, 2018, by adding 7 days, the result is March 18, 2018, and then decreases 3 months which means December 18, 2017, and adds another 1 year. So, in this case, the Preceded Date of Delivery 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 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 is already with the organs formed and your body begins to concentrate, especially in accumulating fat and preparing for the time of birth.