To understand how the gestational week is calculated, it is first necessary to understand the difference between gestational age and fetal age!
Gestational age, first of all, defines the standard menstrual cycle as 28 days, and with this in mind, starts counting from the last menstrual period. The full 40 weeks (280 days) of gestational age is the expected date of delivery.
The gestational age, counting from the union of the sperm and egg, reaches 38 weeks (266 days) as the expected date of delivery.
Therefore, during the first two weeks of gestational age, the embryo is not yet present, the follicles are still developing during these 14 days and ovulation has not yet occurred, but the calculation of the gestational weeks has already begun.
The calculation of gestational age and gestational age involves several key words: date of last menstruation, menstrual cycle, follicular phase, ovulation date, fertilization date, and luteal phase.
Only the date of the last menstrual period and the menstrual cycle are known conditions, the luteal phase is usually a fixed 14 days, the others are calculated by projection. And the day of ovulation and the day of fertilization are very close to each other, so by default they are the same period.
Menstrual cycles can be long or short, and calculating the gestational week by the date of the last menstrual period to measure the age of the fetus is actually a rough alternative to the calculation of gestational age. This is because we can only know exactly the date of the last menstrual period, not the date of sperm-egg union. The international practice is to calculate the gestational week from the last menstrual period, and doctors usually only describe the gestational age, not the gestational age.
If you want to get to the root of the problem, look here, you can figure it out yourself, but don’t bother the doctor!
Follicular phase = menstrual cycle – luteal phase = menstrual cycle – 14 days
day of fertilization ≈ ovulation = date of last menstruation + follicular phase
The normal regular menstrual cycle is defined as 28 days, and our accounting of the gestational cycle is standardized on a 28-day menstrual cycle.
Assuming that woman A has a 28-day menstrual cycle and her last menstrual period is February 1, 2016, the starting point for her gestational week calculation is February 1, 2016, and 280 days later is her due date, which is November 7, 2016. The starting point of her gestational age calculation is February 1, 2016 + follicular phase = February 1, 2016 + (menstrual cycle – 14 days) = February 1, 2016 + 14 days = February 15, 2016.
Assuming that Woman B has a 35-day menstrual cycle and her last menstrual period is January 25, 2016, it would be inaccurate to use the date of her last menstrual period to calculate her gestational week because her ovulation date is actually still February 15, 2016. This is because ovulation day = date of last menstruation + follicular phase = date of last menstruation + (menstrual cycle – luteal phase) = January 25, 2016 + (35-14) = January 25, 2016 + 21 = February 15, 2016. This accounts for the fact that Woman B’s due date is still November 7, 2016.
If the menstrual cycle varies from 26 to 30 days, with one or two days of fluctuations of this kind, do not bother, why bother brain warp for the difference of one or two days? (OCD go home to find a corner squatting slowly think, do not bother me, I also do not understand!)
1, why the clinical only say the gestational age, not the gestational age?
Because we can not know the specific date of fertilization, but there is a way to know the specific date of the last menstruation. Clinically, doctors write records and strive to record the most realistic and objective data, the date of the last menstrual period, which is real and objective, and the menstrual cycle is real and objective. We use the date of the last menstrual period to calculate, the record will write “xxx weeks of menopause”, so that the information is accurate and verifiable. If we record “gestational age xxx weeks”, what is the basis of gestational age calculation, and whether it is trustworthy, cannot be verified! But record the objective date of the last menstrual period, we can calculate the gestational age according to this time at any time, because this is the first-hand information.
2.Why does the due date written by the doctor during the card construction not match with the due date stated by the doctor in the hospital?
There is an easy way to calculate the due date, which is {month + 9 (or -3), date + 7}. For example, if the last menstrual period is February 1, month + 9 = November, date + 7 = 8, and the due date is November 8. This algorithm comes like this, 280 days = 3 months 31 days + 6 months 30 days + 7 days = 3 * 31 + 6 * 30 + 7 days. Obviously this algorithm is not precise, because February 28 days, July and August two consecutive months are 31 days, this algorithm comes up with a due date, and 280 days precisely calculated due date may have a difference of one or two days. However, this does not affect anything, there is a very simple way to correct it, because 40 weeks is a whole number, the week of the last menstruation and the due date should be the same, for example, if the last menstruation is Monday, the due date must be Monday. Then, if the due date is not a Monday, the actual due date is the Monday closest to it.
For example, if the last menstruation is on Monday, February 1, 2016, and the due date calculated with {month + 9 (or -3), date + 7} is Tuesday, November 8, 2016, obviously this due date is not accurate, one is Monday and the other is Tuesday, then the accurate due date is the Monday closest to November 8, 2016, which is November 7, 2016. No you can count on your own fingers, from February 1, 2016, to November 7, 2016, is it exactly 280 days in between.
Therefore, for those who have a very regular menstrual cycle, whether you have a 28-day or 40-day cycle, the pregnancy week accounting is easier to deduce accurately through the last menstrual period.