Births from A to Z
How to determine the date of birth?
Every future mother wants to know the date of X-day, when the birth should begin. However, not all pregnant women clearly imagine what to orient themselves in the calculations. How can I independently calculate the expected date of birth and what does the "mathematical error" depend on?
Elizaveta Novoselova obstetrician-gynecologist, Moscow
We calculate the date of delivery for the last menstruation
The most common method for determining the expected date of childbirth (PDR) delivery is the countdown from the first day of the last menstruation: to this number it is necessary to add 40 weeks, or 280 days, - this is how much the pregnancy lasts on average. It is possible to count without a calendar: it is enough to take 3 months from the first day of the last menstruation and add 7 days to the received date.
Why do doctors focus on the last menstruation? After all, conception in this period if not impossible, then at least it is least likely! The fact that the monthly - a kind of evidence that at the moment pregnancy has not come: because menstruation is the end of another, "unsuccessful" from the point of view of fertilization, the cycle. Since the next menstruation in a woman was no longer, it is logical to assume that it was in the cycle that followed the last monthly, and pregnancy occurred.
The most probable moment of conception is ovulation - the exit of a mature egg from the ovary. Usually this event occurs on the 12-16th day of the cycle, therefore, by the 40 weeks of gestation counted from the first day of the cycle, another 14 days, or 2 weeks, are added.
However, this method of calculating the date of birth has its own errors: there is a small chance that the conception occurred yet ... before the last menstruation. This is possible in the event of failure of ovulation due to stress, hormonal intake, climate change, significant physical exertion or disease. And then the day of ovulation is shifted from the middle of the cycle (as should be normal) closer to the next monthly.
If the conception took place within 1 week (or less) until the next expected menstruation, the woman's body may simply not be in time to adjust to the regimen of pregnancy, and the monthly will begin.
Conception on the eve of menstruation, of course, poses a risk to the embryo and significantly reduces the chances of maintaining the pregnancy, but the probability that it will not be interrupted is still there! During menstruation, the endometrium, a layer of the mucous membrane, is rejected in the uterus, which in the case of fertilization must provide the embryo with protection, nutrition and oxygen. However, the embryo at this moment may not be in the uterus, but in the fallopian tube, where no phenomena of rejection occur and, accordingly, nothing threatens it.
From the moment of fertilization to embryo entering the uterine cavity passes an average of 4-6 days, and before its implantation (attachment to the uterine wall) - 7-14 days. During this time, menstruation can be completely completed, and the uterine wall will begin to be covered with a new endometrium, which is necessary for successful implantation.
Given the probability of pregnancy before the last menstruation, from 40 weeks, counted from the first day of the last cycle, take 14 days, or 2 weeks.
Thus, it appears that the estimated date of delivery is 40 weeks from the first day of the last menstruation, plus or minus 2 weeks. According to this calculation, timely, or, as it is customary to say in obstetrics, "urgent" (from the word "on time"), birth can begin any day from 38th to 42nd week, counted from the first day of the last menstruation. The most probable date is the "golden mean" of this period - 40 weeks.
But on this inaccuracy of the "menstrual" method of counting does not end. The fact that the duration of gestation depends on the rate of metabolic processes in the body of a woman.
The intensity of metabolic processes that are directly dependent on hormonal regulation can be judged by the duration of the menstrual cycle: as is known, it usually lasts 28 days, or 4 weeks, but in 25-30% of women its duration differs from the average.
In a healthy woman, the cycle length can vary within a week, that is, a normal regular menstrual cycle should not be shorter than 21 days and longer than 35 days, large deviations indicate dysfunction (hormonal imbalance).
An unchanged (regular, constant) menstrual cycle of 21-23 days duration indicates a high intensity of processes controlled by the hormonal background, and, since pregnancy is also hormone dependent, a woman with a shortened cycle may normally have a gestation period of less than 40 weeks.
Similarly, if menstruation regularly came in 33-35 days, then the woman has an extended cycle, which indicates a low intensity of metabolism, in which the body may need more time to fully bear the fetus than the classical 40 weeks.
Short-cycle holders are recommended to count from the first day of the last menstruation not 40, but 39 weeks, and women, whose cycle is lengthened, should be added to the 41-th week. In this case, the rule "plus or minus 2 weeks" remains in effect, so that during the cycle of 21-23 days delivery from the 37th to the 41st week will be considered timely, and pregnancy - full. With a menstrual cycle of 33-35 days, the baby will be born between 39th and 43rd week.
PDR is calculated by doctors in order to monitor the preparation of the maternity organism for childbirth at the right time and not miss the signs of overstretching, a pathology in which the pregnancy continues despite the fading of the placenta function. This violation is diagnosed by obstetric examination, ultrasound and CTG. If there is no repetition, pregnancy can last without harm to the baby and mother and more than 40 weeks.
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Determine the date of delivery according to the size of the uterus
There is also a method for calculating the length of pregnancy and the expected date of birth, such as measuring the size of the uterus with the help of obstetric examination and palpation (palpation). In the first trimester this organ is still entirely located in the pelvic cavity, therefore, to determine the term, the doctor must perform a vaginal examination. But already from the 12th week of bearing the fetus, the upper part of the uterus (its bottom) begins to protrude above the pubic articulation and is easily felt by hand during external examination. From this moment, the gestation period is calculated by the height of the standing of the uterine fundus, that is, by the level of the location of its upper part. By the 4th week the uterus reaches the size of a hen's egg, and at 8 weeks it grows to the size of a goose egg; however, it can only be determined by a doctor during an obstetric examination (vaginal examination). At 12 weeks, the uterus corresponds to the size of the newborn's head, and further, during the entire remaining period of gestation, the gestation period can be judged by the height of the standing of the uterine fundus.
Calculate this figure can and the mother herself: it is enough to put your hand to the most prominent point of the tummy.
At 16 weeks the bottom of the uterus is located exactly in the middle between the navel and the pubis, or four transverse fingers above the upper bikini line.
By the middle of pregnancy, at 20 weeks, the uterus's bottom is on two transverse fingers below the navel, at this time the tummy is already noticeably protruding.
At 24 weeks the bottom of the uterus is located exactly at the level of the navel.
At a period of 28 weeks, the upper abdomen is above the navel by 2-3 transverse fingers.
At 32 weeks the bottom of the uterus rises to the middle of the distance between the navel and the xiphoid process (the lower edge of the sternum).
By the 36th week, the upper border of the abdomen rises to the xiphoid process and costal arches, this is the highest location of the uterus during the entire period of pregnancy.
After the 38th week, the bottom of the uterus begins to descend, and by 40 weeks it is again at the same level as it was at 32 weeks, that is, in the middle between the navel and the xiphoid process.
You can also more "professionally" calculate the length of pregnancy by the size of the abdomen, for this future mother will need a centimeter tape. A zero centimeter mark should be placed in the middle of the lone articulation (along the upper bikini line), and the free end of the tape leads in the direction of the navel to the most protruding point of the bottom of the uterus. The measurements should be lying on the back. The result corresponds to the height of the standing of the uterine fundus, it is possible to determine the gestational age and calculate the expected date of delivery:
- 16 weeks - 6-7 cm;
- 20 weeks - 12-13 cm;
- 24 weeks - 20-24 cm;
- 28 weeks - 24-28 cm;
- 32 weeks - 28-30 cm;
- 36 weeks - 32-34 cm;
- 40 weeks - 28-32 cm.
However, this method of calculation can not be relied on completely and be 100% sure of it, because here all the factors that affect the increase in the size of the abdomen of the future mother are not taken into account. In the case of polyhydramnios, multiple pregnancies, miscarriage of the fetus, the shape and size of the abdomen may differ significantly from the stated norms. In addition, it makes no sense to focus on the height of the standing of the uterus bottom with a narrow pelvis or excess weight of a future mother, as the size of the abdomen in both cases will be greater than it should be according to the time.
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DAW by ultrasound
The most effective and modern way to calculate the expected date of delivery is the ultrasound method. With the help of ultrasound it is possible to determine the gestational age (gestation of the organs and tissues of the embryo), the degree of maturation of the placenta, the size and development of the fetus. The possibility of diagnosing several criteria at once makes the ultrasonic method the most accurate, however, it also has inaccuracies that must be taken into account.
The main ultrasound criteria for determining the period - the stage of gestation, the formation of the organs and systems of the embryo - are relevant in the first half of pregnancy. By the 20th week of bearing a child, the laying of his organs and tissues has actually been completed, and the date can be judged mainly by the size of the fetus and the degree of maturation of the placenta.
However, unlike the factors of gestation, these criteria are not so reliable and unchanged.
In the computer program evaluating the results of ultrasound measurements, the norms of the size of the fetus for each period of its bearing are laid down. But after all the program uses average values, therefore, if the fetus is large or, on the contrary, tiny, the gestational age, established on the basis of its anthropometric parameters (growth and estimated weight), will differ from the real one. That is why in the conclusion of ultrasound at a later date after the phrase "corresponds to the term of pregnancy" the doctor always indicates in parentheses: "according to the size of the fetus".
The same applies to diagnosis of the degree of maturity of the placenta: it is customary to single out the zero, first, second and third, and each degree corresponds to a certain period of bearing of the child. But with the development of some complications of pregnancy, such as late toxicosis, insufficiency of placental blood flow, placenta (nonspecific inflammation in the placenta), intrauterine infection, premature ripening of the placenta occurs, and then the degree of its maturity can significantly outstrip the actual duration of gestation.
It happens that the future mother, receiving the next result of ultrasound, in which the pregnancy period does not correspond to the real one, is experiencing that she "was again moved to the date of birth". In fact, premature maturation of the placenta and an increase in the size of the fetus does not in any way affect the duration of gestation and the expected date of delivery. In the results of the study, the doctor only ascertains to what date of pregnancy the given sizes of the baby and the degree of maturity of the placenta correspond.
Comprehensive calculations of the expected date of delivery (PDR)
For the most accurate calculation of the date of delivery it is necessary to use all the listed methods taking into account all possible errors. The PDR will be the "arithmetic mean" of all the numbers obtained.
It is still important to remember that in the abbreviation "PDR" the key word is "supposed", that is, the calculations do not mean that it is on this particular day that a woman must certainly give birth. This date, corresponding to the term of 40 weeks of pregnancy, is the "golden mean" in the interval from the 38th to the 42nd week, but timely delivery can occur on any day of this period.
Date of birth: we count on the first stirring of the fetus
The estimated date of birth can be calculated, focusing on the moment when the future mother for the first time felt the stirring of the baby. Only this method is very inaccurate, since it is based solely on the subjective perception of a woman. During the first pregnancy, many inexperienced future mothers often confuse the perturbations with intestinal peristalsis, which is aggravated by the pressure of the growing uterus. It is believed that a woman, first preparing to become a mother, begins to feel a distinct impact on the fetus from the 20th week of pregnancy. And the women who are mated often quite often begin to distinguish the first weak shocks of the fetus already from the 14th-16th week, as they remember these sensations from previous pregnancies.
The time at which future mothers can clearly recognize the movements of the fetus depends not only on the experience: the physique of a woman and the particular features of this pregnancy are of no small importance. For example, ladies hypersthenic type (large, full) often begin to feel the movements of the baby not earlier than the 22nd week, this is due to poor conductivity due to excess fat tissue or muscle mass of the body. And on the contrary, graceful and tiny mums can also feel baby tremors in the first pregnancy before 20 weeks due to a deficiency of the subcutaneous fat layer of the anterior abdominal wall.
With insufficient amount of amniotic fluid, large fetal size or multiple pregnancies, the perturbations can also be felt already earlier than 18 weeks. And with a tendency to polyhydramnios, as in the case of excess of subcutaneous tissue in the mother, the fetal tremors are extinguished by the fertile waters, and the perturbations begin to be clearly heard later than usual.