At the end of this article, we have a pregnancy calculator so you can calculate the time of pregnancy, when it will be the probable date of birth and estimate what was the day when conception occurred. But before we will explain all about the ways to calculate gestational age. It is common knowledge that a pregnancy lasts an average of 9 months. However, as soon as the pregnant woman goes attend prenatal visits she learns that her obstetrician does not count the time of pregnancy in months but in weeks. For those who are not accustomed to this way of counting gestational age everything seems more complicated. Would not it be easier if the doctor say that she is in the fourth month of pregnancy?
In fact, doctors do not have time agreed pregnancy weeks only to confuse the head of pregnant future. There is logic behind this decision. Changes in the body of the woman and fetus are many and occur relatively rapidly, especially in the first half of pregnancy. In one month, take place dozens of major changes, and counting in weeks allows us to define more precisely the time of each. For example, (fertilized egg) is implanted in the uterus wall during the 3rd week, the heart is formed on the 4th, the embryo becomes visible in the ultrasound on the 5th, the eyes appear on the 6th and the future baby starts moving in the 7th week (the mother does not notice because it is too small). If we gestation in months, we would not be as accurate instantly assess the time of such changes.
Another confounding factor is the fact that the Gregorian calendar we use does not establish every month with the same number of days. There are months of 28, 29, 30 or 31 days. Therefore, to be synchronized with the weeks, we adopted the lunar month, which lasts 28 days or 4 weeks. So, weeks, months and trimesters of pregnancy are as follows. Some authors consider that the 3rd trimester begins from the 27th week of pregnancy. This, in reality, has little clinical importance because medical decisions are based according to weeks pregnant and not the quarter.
The most correct way to calculate the time of pregnancy is through the day of conception. However, it is very difficult to define exactly what the day that the sperm found his egg was. It is important to remember that although only you had sexual intercourse in the month they became pregnant, this does not mean that the conception occurred this day. Sperm can remain viable in the female reproductive system for 5 to 7 days, depending on the viability of the same. So if you had sex on the 16th, but only ovulate on the 19th, conception occurred only three days after intercourse
If you had more than one sexual relationship in the same month, it is almost impossible to find out what was the exact date of fertilization. As the egg is viable for 24 hours and sperm up to 7 days (usually 3 to 5) if you got pregnant in a month that had sex on days 12, 14, 17 and 19, this means that the fertilization of the egg could have produced between 12 and 26 days a month. Therefore, since it is almost impossible to know the exact day that fertilization has occurred, to the simplest calculation was appointed the first day of pregnancy as the first day of the last menstrual period. It is very common that doctors refer to this day as DUM (acronym for Day LMP).