Weeks are commonly used to measure pregnancy, but many expectant parents are unaware of the formula. Questions about the precise start date of the pregnancy and the reasons behind the discrepancies in doctor’s estimates may arise from this.
Some of this confusion may be reduced by knowing how pregnancy weeks are calculated. Depending on whether you’re counting from the date of conception or the last menstrual cycle, there are a few different methods to calculate it. Every technique has a specific purpose for being employed.
You can better track the progression of your pregnancy and be prepared for each stage by understanding how these calculations are made. Making sure everyone is on the same page during communication with healthcare providers is also beneficial.
- Counting methods
- Obstetric calendar
- Ultrasound method
- Gestational age
- Online calendars and counters
- Video on the topic
- Pregnancy period. How to calculate? Obstetric period. Ultrasound screening. EDD.
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- Pregnancy. How to calculate the term
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Counting methods
The counting of pregnancy weeks is very important for both the future mother and doctors, first of all, because it allows you to track the development of the baby and the progression of pregnancy, comparing them with general standards. Also, the definition of the exact deadline will help to calculate the day on which the birth presumably begins. All examinations, tests, screening, all tables of analyzes and data of ultrasound in gynecology and obstetrics are compiled in obstetric weeks, which usually differ significantly from actual. So that there are no discrepancies and confusion, a woman is recommended from the very beginning to get used to counting the time as doctors do. Will the term be accurate, it’s difficult to say. Most likely not, because the obstetric method is averaged, but it is it that is the most optimal. There are no exact methods because it is impossible to determine the time of conception even to the exact day. A woman is fertile for only 24-36 hours throughout the entire cycle. This is her ovulation period. This is how long a mature egg remains viable. Sperm may already be in the genital tract by the time the oocyte leaves the follicle, or they may reach the egg later, but again no later than a day after its release. If the conditions are met, conception occurs.
For the vast majority of women, ovulation happens in the middle of the cycle, around day 14 of a 28-day cycle. However, it might change and happen later. A number of factors, including stress, medication, exhaustion, colds, and a host of others, can influence this and are theoretically impossible to fully account for.
Thus, a woman who claims to know exactly when the baby was conceived may be mistaken. If it leads a normal rich sex life, then in principle it cannot know the exact moment of conception. Gynecologists, despite the whole development of modern medicine, also cannot establish the day and hour of conception, and therefore use a more simplified system for all the calculation of obstetric weeks. Of course, the first day of the cycle and the onset of menstruation cannot be the first day of pregnancy. Nevertheless, it is believed that a mature egg at the time of ovulation has already passed about 14 days of its development. In this regard, the discrepancy between obstetric and embryonic time can be 2 weeks. Also You can determine the gestational age according to the ultrasound is the most accurate way, If, of course, take into account the date of menstruation. You must always evaluate these two parameters together, since the size of the fetus can be more or less than the norm for the same time for different women for different women. Let"s step by sharing we will consider the main methods.
Obstetric calendar
It is this method that is considered the most correct, despite what causes inexperienced pregnant women sincere bewilderment – is it possible to be pregnant, when there was no sexual intercourse yet? You can. The first two weeks of the obstetric period are the first part of the menstrual cycle, preceding ovulation, it gives development to the egg itself, the second two weeks – pregnancy is already actual, but not yet obvious. Only at the end of the 4th week of pregnancy there is a delay, when you can do tests, take blood tests for hCG. If they are positive, the doctor makes an appropriate decision based on the date of the last menstruation – 4 weeks of pregnancy. In fact, from the moment the baby begins to exist, about 2 weeks have passed by this time. Such weeks of development are called embryonic.
Pregnancy lasts for just 40 obstetric weeks. The 40th week ends on the day of the birth. However, giving birth at any point between the full 37th and 41st week and 6 days will be accepted as perfectly normal.
Long before humans invented the ultrasound and undoubtedly before online accounters and calculators, pregnancy was regarded as obstetric.
- (first day of menstruation minus three months) + 7 days;
- (first day of menstruation + 9 months) + 7 days;
- first day of menstruation plus 280 days.
Which one to use will be up to the doctor alone. And when the expectant mother visits the antenatal clinic, he will register her there.
There are precisely ten obstetric months during pregnancy, split into three trimesters. One obstetric month is equivalent to four obstetric weeks. We now understand why the time from the moment of conception is roughly two weeks shorter than the obstetric period.
When the doctor states that the patient is eight weeks pregnant after examining her and twirling the calendar in his hands, this is not an error; rather, it is what is meant to happen: Eight obstetric weeks correspond to six weeks from the time of conception, or precisely four weeks from the start of the delay.
It can occasionally be confusing to determine whether the pregnancy weeks are calculated using the date of conception or the first day of the last menstrual cycle. Although there may be a small discrepancy in the conception date, doctors usually employ the menstrual cycle method for consistency. Expectant parents can better track their pregnancy and prepare for each stage by being aware of these differences.
Ultrasound method
Because Uzi’s equipment is set up for embryonic weeks, an ultrasound reading of seven weeks indicates that the pregnancy is precisely seven weeks based on the parameters of the embryo (fetus), not nine, as some people believe, since the term must be added to the actual time of two obstetric weeks.
By providing results in weeks and days, such as 7 weeks and 5 days or 7 weeks and 3 days, modern ultrasound scanners enable you to confirm the pregnancy period that obstetricians have already determined. Accordingly, the size of the embryo represents a period of 7 weeks and 5 days for one woman and 7 weeks and 3 days for another. Since no two people are exactly alike, the size of the embryo varies even within the same time period. The period indicates that both babies are developing normally, as the variance of up to one week is within acceptable bounds.
When a woman has an irregular or broken cycle and cannot recall the exact date of her last menstrual cycle, she may benefit from using the ultrasound method to determine the exact period. Furthermore, if there is no menstrual period and lactation or the transfer of a defrosted embryo in a delayed cycle, the date of menstruation may not be useful in establishing the pregnancy period.
It is crucial to understand that the early stages of an ultrasound examination are the only times when it is possible to determine the term with the greatest accuracy. This is because, up until 9–10 weeks before delivery, all embryos grow at roughly the same rate during the embryogenesis stage.
Next, heredity and a person’s unique developmental trajectory come into play: a baby born to short, stocky parents will grow up to be small and stocky, but another baby born to tall parents will show remarkable growth and weight indicators from the middle of the pregnancy.
Ultrasound data can be used to pinpoint the date of conception up to 11 or 13 weeks in advance. It is during this period that the first prenatal screening is performed, with the aim of both adjusting the timing and identifying any potential chromosomal abnormalities in the fetus.
Gestational age
This is another obstetric term that causes confusion for many women, both experienced and first-time mothers expecting the birth of their first child: when does gestation start, what is it, and why is it even considered?
This term obscures the already-familiar embryonic phase, which is the time between the union of the mother’s and father’s germ cells and the birth of a new, independent child. The individuals who underwent IVF (in this case, doctors know not only the day, but also the hour, when and how the fusion of the egg and sperm occurred, because it happened in a test tube) and those who underwent insemination on the day of ovulation, confirmed by an ultrasound protocol, are the only ones who know the precise gestation period.
This in no way implies that post-IVF pregnant women should calculate their period using its gestational equivalent. Like in other cases, the pregnancy will be measured in obstetric weeks starting on the first day of the last menstrual cycle, if the woman is aware of it, or based on ultrasound data, but strictly speaking in obstetric weeks.
Online calendars and counters
To find her expected date of birth, or EDD, a modern woman does not have to perform laborious and time-consuming calculations with a pencil and calendar, adding or subtracting months. There are many distinct types of counters and calendars available today. The results include the current period, the anticipated day of birth, the month and trimester ordinals, and even the probability of having a boy or girl (everything except this point is true; the last point is for fun and comes true with a 50/50 probability). All you have to do is enter the date of your last menstruation and the length of your menstrual cycle.
Certain counters have more sophisticated features than others; in addition to counting everything, they can give specifics like the baby’s height and weight, development norms, and how it is growing week by week. There are no costs at the counters.
| Method | Explanation |
| Gestational Age | This counts from the first day of the last menstrual period (LMP), commonly used by doctors. It assumes ovulation occurs about two weeks after the LMP. |
| Fetal Age | This calculation starts from the actual conception date, which is typically two weeks later than the gestational age. It"s used less often in clinical settings. |
| Why Calculations Differ | The gestational age includes those two extra weeks before conception, while fetal age counts only from the moment of conception. |
| Why Gestational Age is Common | It"s easier to track since most women know the start of their last period, but not the exact conception date. |
It’s critical to comprehend pregnancy week counting in order to monitor the health of the mother and the developing child. Anticipating each milestone’s arrival date facilitates planning for the future and guarantees on-time health examinations.
Although there may be small variations in the methods used to calculate weeks, the most popular method begins on the first day of the most recent menstrual cycle. This provides parents and healthcare providers with a clear, uniform timeline.
The ultimate objective is to closely monitor a pregnancy and guarantee the mother and child receive the best care possible. Parents can feel more prepared and confident during this exciting time by understanding how and why calculations may differ.
