At the end of this article we have a pregnancy calculator you can calculate the time of gestation, when will be the probable date of birth and to estimate which was the day the conception.
But before resorting to the calculator, it pays to understand how it is done the calculation of gestational age.
It is common knowledge that a pregnancy lasts an average of 9 months. However, as soon as the pregnant woman passes to attend prenatal consultations, she discovers that the your obstetrician doesn’t count the time of pregnancy in months, but in weeks. For anyone who’s not used to this way of counting the gestational age, everything seems more complicated. After all, 17 weeks is how many months again? Wouldn’t it be easier if the doctor say that the woman is in the fourth month of pregnancy?
In fact, the doctors have agreed not to tell the time of pregnancy in weeks just to confuse the head of expectant mothers. There is a logic behind this decision. The changes in the body of the woman and the fetus are many and occur relatively quickly, especially in the first half of pregnancy. In a single month, there are dozens of important changes, and counting in weeks allows us to define with more accuracy each time.
For example, the Zygote (fertilized egg) implants itself in the wall of the uterus in the third week with maternity trousers, the heart is formed in the 4th, the embryo becomes visible on the Ultrasound in 5th, the eyes appear in the 6th and the future baby starts moving in the 7th week (the mother doesn’t notice because he’s still too small). The total gestation in months, would not be so precise in time to assess the time of such changes.
Another confounding factor is the fact that the Gregorian calendar that we use do not set every month with the same number of days. There are months of 28, 29, 30 or 31 days. Therefore, to be synchronicity with the weeks, adopt the lunar month, which lasts 28 days or 4 weeks. Thus, the weeks, months and quarters of pregnancy are as follows:
1 to 4 weeks gestation: 1 month
5 to 8 weeks of gestation: 2 months
9 to 12 weeks of gestation: 3 months
13 to 16 weeks of gestation: 4 months
17 to 20 weeks of gestation: 5 months
21 to 24 weeks of gestation: 6 months
25 to 28 weeks of gestation: 7 months
29 to 32 weeks of gestation: 8 months
33 to 36 weeks of gestation: 9 months
37 to 40 weeks of gestation: 10 months
Some authors consider that the third quarter starts from the 27th week of pregnancy. That, in fact, has little clinical relevance, because medical decisions are based according to the weeks of gestation, and not with the quarter.
How To Calculate The Weeks Of Pregnancy
The correct way to calculate the time of pregnancy is through the day of conception. However, it is very difficult to define what exactly was the day that the sperm found your egg. It is important to remember that even if you only had a single sexual intercourse in the month you got pregnant, that does not mean that the conception took place this day. The sperm can remain viable in the female reproductive system for 5 to 7 days, depending on the vitabilidade. So, if you had sex on day 16, but only ovulated on day 19, the conception occurred only 3 days after copulation.
If you have had more than one sexual relationship in the same month, there even is it’s almost impossible to figure out what was the exact day of fertilization. As the egg is viable for 24 hours and the sperm for up to 7 days (usually 3 to 5), if you got pregnant in a month which experimented on days 12, 14, 17 and 19, it means that the fertilization of the egg might have occurred between 12 and 26 days of the same month. In other words, an immense range of 2 weeks.
Therefore, as it is almost impossible to find out the exact day that fertilization has occurred, in order to make the simplest calculation, was designated the first day of pregnancy as the first day of the last menstruation. It is very common to physicians refer to this day as a (which stands for date of last menstrual period).
To Facilitate Understanding, Let’s Imagine A Fictional Situation:
A woman with a few weeks of menstrual delay decided to do a pregnancy test which came back positive. Today is March 09 and the first day of your last menstrual period (a) was on 23 January. For the purpose of calculating gestational age, we consider the 23 day of January as the first day of pregnancy, even though she’s not being effectively pregnant at that time. So, the first week of gestation will of the day 23/01 the day 29/01. The second week will of the day 30/01 day 05/02, and so on. Following this calculation, we have come to the conclusion that on 09 March our imaginary pregnant would be 6 weeks and 4 days of gestation.
In the image below, we split the weeks in different colors to facilitate understanding.
Watch out for the fact that ovulation and therefore conception just occurred between the 2nd and 3rd weeks after menstruation. This means that the way we tell time to pregnancy, in the first two weeks, the woman wasn’t really pregnant. Therefore, when we say that the patient is in the 10th week of pregnancy, in real life she’s around the eighth week. But this discrepancy does not cause any problem. As every woman ovulates on a different day of the menstrual cycle, use the first menstruation facilitates the standardization of the time of pregnancy, even if it means a few days apart in the real time of gestation.
And If The Woman Doesn’t Remember Exactly When Was Your Last Period?
Several situations can take the wife not to remember the exact day of your menstruation. For example, women with very irregular menstrual cycle, women who just discover the pregnancy with several weeks of pregnancy, women who had escaped bleeding in early pregnancy and the confused with menstruation, etc.
In these cases the transvaginal ultrasound is used to estimate the time of pregnancy. From the fifth week of pregnancy, the embryo can be identified by ultrasound.This test has a very high accuracy if it is done in the first quarter.
Until the first 20 weeks of pregnancy, all fetuses have more or less the same size. So, simple biometric measures, such as the size of the femur bone, head circumference, length cephalus-capacity and waist circumference, helps the doctor to establish the gestational age. After the 20th week of pregnancy, genetics kicks in and each baby happens to grow in a different pace. Thus, from this moment on, it becomes more difficult to say the exact gestational age based on the size of the fetus.
Even women who know your point a, the calculation of gestational age is accomplished by fetal ultrasound to confirm the estimate. The calculation done by ultrasound is more reliable than the date of the last menstruation.
To learn more about obstetric ultrasound, read: ultrasound in pregnancy.
Size Of The Uterus
The size of the uterus is another way to estimate the time of pregnancy. Before the advent of ultrasound, this was used to estimate gestational age in women who didn’t know your DUM. Up to 12 weeks of pregnancy, we weren’t able to palpate your uterus. From the 12th week we can palpate your uterus just above the symphysis pubis (front of the bone of the pelvis). With 16 weeks, the uterus can be palpated halfway between the pubic symphysis and the navel. With 20 weeks of pregnancy, the uterus is at the navel.
How To Calculate The Day Of The Birth
When calculated by DUM, a normal pregnancy (or term) is one that lasts between 37 to 41 weeks. If the baby is born before the 37th week, he is considered premature. If you were born after the 42nd week, it is called post-term baby.
In most cases, babies are born around the 40th week of pregnancy. Therefore, when the midwife tells you the estimated date for delivery (DPP), he simply told 40 weeks after your DUM. A quick way to estimate the DPP without counting the days on the calendar is through the formula of Naegele:
1 – take the first day of your last menstrual period (a) and add seven days.
2 – Decrease three months.
3rd – Some a year.
A = 05 may 2014
Some 7 days = 12 may 2014
Subtract 3 months = February 12 2014
Some a year and get the DPP = 12 February 2015
A = January 2014 02
Some 7 days = 09 January 2014
Subtract 3 months = 09 October 2013
Some a year and get the DPP = 09 October 2014
Obviously, this is only an estimate. The goal is not to guess the day of delivery, but, Yes, inform the mother of the season likely the birth, if the pregnancy go to the 40 weeks. In fact, only about 5% of babies really are born in estimated date at the beginning of the prenatal care.
Use our pregnancy calculator to know your dates and when will your DPP.
Use this tool to calculate how much time of pregnancy you have, what will be the likely day of childbirth and to estimate in that day, approximately, there was fertilization.
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