The importance of obstetric gestational age
The period calculated from the 1st day of the last menstruation is called the obstetric (gestational or menstrual) period. With this calculation, births that occur approximately on the 280th day or 40th week from the start of the term are considered normal. This corresponds to 9 and 1/3 calendar months.
According to the obstetric period when managing pregnancy, doctors:
- The estimated date of birth (ED) is determined.
- A screening program for the pregnant woman is prescribed.
- Monitor the development of the fetus and identify deviations from the norm if they appear.
- Document the date opening maternity leave, starting from the 30th obstetric week.
Obstetric method for calculating gestational age: what is it, how is it calculated?
The obstetric calculation method is a classic way of determining the duration of pregnancy, based on information about the first day of the menstrual cycle in which conception occurred. The first day of pregnancy is the date when your last period began.
For example, a woman went to the gynecologist on April 4 because her menstruation was delayed. She told the doctor that her last period began on March 1st. The obstetric pregnancy period is considered from March 1. Accordingly, as of the date of treatment, she has 5 obstetric weeks.
For many women, this method seems illogical, since for conception it is necessary for ovulation to occur. However, the obstetric counting method was developed long before reproductive processes were studied and methods for determining ovulation became known. The only reference point for the doctor was menstruation.
With the development of science, the technique found justification in the reproductive mechanism itself. During the first period of the menstrual cycle, preparation for fertilization occurs, and after ovulation, the body produces hormones to maintain pregnancy in the event of fertilization.
How accurate is it?
In order not to get confused, it is better for a pregnant woman to immediately learn how to count as is customary with specialists.
Yes, the calculation will not be absolutely accurate. But this method is optimal - there are no more accurate alternatives for it.
However, there are cases when it is impossible to determine the obstetric period correctly:
- In the physiological absence of menstruation during lactation.
- When a woman has endocrine or other disorders accompanied by irregular cycles.
- If the patient has a long menstrual cycle (from 35 days).
- In the case of intermenstrual bleeding, which disrupts the biological rhythm and confuses the woman in her calculations.
Ultrasound examination during pregnancy
Throughout the time the fetus is in the womb, the woman regularly undergoes various studies. One of the most important is ultrasound.
If you did not have to do an ultrasound early on to determine the date of conception, then for the first time you will have to face such an analysis from 10 to 14 weeks. It is then that you can determine as accurately as possible the time during which pregnancy develops. Also, based on this study, the expected date of birth of the baby is established.
As pregnancy progresses, the woman will have to undergo this examination at least twice more. This will happen approximately from 20 to 24 weeks and from 30 to 34. It is worth saying that these studies may have a large error. The date of birth of the child established according to them may differ from the real one by 2-3 weeks. This is why it is so important not to miss the first screening and do an ultrasound on time.
Differences between obstetric and embryonic periods
The embryonic (true) period of pregnancy is the period starting from the time of conception. The time of ovulation and conception is taken to be the middle of the cycle, which is equated to 28 days.
When calculating the embryonic period, 2 weeks are added to the first day of the last menstruation. The difference with the obstetric period is about 14 days. But usually ovulation occurs between 12 and 18 days from the start of menstruation.
The main difference is the impossibility of determining the real time of conception. The egg lives and is ready for fertilization only 24–36 hours after ovulation. Spermatozoa remain alive and functional in the female reproductive tract for 5–6 days.
Determination of period by menstruation
This rather old method is considered the most common. It is still used as a primary option by both specialists and women without medical training.
In order to determine the approximate duration of pregnancy, it is necessary to count the number of days that have already passed from the last date of the critical days. This will be your deadline. In addition, in a similar way it is possible to find out the approximate day of birth. To do this, add another two hundred and eighty days to the day you calculated earlier. It is also possible to use the so-called Nägel formula. If you count according to it, then three months are subtracted from the date of your last period, and then one week is added to the result.
- Pros. Availability, no cost. You can perform the calculations yourself.
- Minuses. This method is not suitable for those women who suffer from irregular, periodically changing cycles. The method is inaccurate.
How to independently calculate the obstetric period
When a woman knows exactly the date of her last menstruation, it will not be difficult for her to determine how long she has been pregnant.
Our online calculator simplifies the task, with the help of which you can quickly accurately calculate the gestational age in obstetric weeks and PPD.
You can calculate the date of birth based on the obstetric period using Naegele’s formula:
- first day of last menstruation + 40 weeks;
- first day of last menstruation + 3 months + 7 days.
What's happening
There is very little space in the uterus now, so don’t be surprised if the number of movements decreases. Be sure to listen to your baby's every movement: both excessive activity and complete calmness are not very good signs. Normally, at 40 weeks of pregnancy, you should feel about 10 movements in 12 hours.
Like the mother, the baby is preparing for birth. In order for the birth process to go smoothly, the tiny body produces the hormones adrenaline and norepinephrine, which should reduce pain when the fetus passes through the birth canal.
By the 39th – 40th week of pregnancy, the baby’s blood circulation is established, the digestive system is actively functioning, processing amniotic fluid and dead skin cells. The meconium formed from them will leave the baby’s intestines only after birth. The bones of the skull are still flexible, allowing the fetus to pass through the birth canal without hindrance.
The 40th week of pregnancy should be the final one, but in practice only a few women give birth at the expected time. So be patient and wait while continuing to lead your normal life. In any case, very soon you will hug your baby.
Gestational age and ultrasound
The ultrasound examination period is determined by the biometric parameters of the fetus embedded in the equipment software algorithms. But the parameters of each organism are individual.
Determining the age of the embryo and fetus during ultrasound is more reliable in the 1st trimester. Especially before the 8-12th week.
As the fetus grows, accuracy decreases due to the characteristics of the unborn child's body. Every person is unique. Even among the same parents, children are born with different weights.
The older the embryo (fetus), the more clearly its unique parameters appear. Therefore, gestational age can most accurately be determined using ultrasound in the early stages.
On average, if the pregnancy proceeds normally, the due date and biometric parameters of the fetus, by which the equipment gives its age, coincide.
However, sometimes the obstetric period does not coincide with ultrasound indicators. If the obstetric period is much longer than indicated by ultrasound diagnostics, additional studies are carried out to determine the developmental delay and its causes.
When an ultrasound shows the age of the fetus is significantly greater than the obstetric term, doctors examine the pregnant woman for the possibility of diabetes mellitus or other endocrine disorders. A number of studies are done to exclude or confirm disorders of the embryonic development of the unborn child.
In any case, the important parameter is the obstetric period - a method proven by many generations.
Other ways to determine the due date (examination, hCG analysis, uterine measurements)
Difficulties in determining the duration of gestation arise in women who cannot accurately name the first day of their last menstruation. This happens in the following situations:
- conception occurred shortly after an abortion or a previous pregnancy before the start of the next menstruation;
- the menstrual cycle is irregular;
- a woman does not track the time of her period.
It is also possible that the expectant mother simply does not remember which day corresponds to the beginning of the regulation. The deadline is set using the following methods:
- Ultrasound;
- gynecological examination;
- blood test for hCG;
- measurements of the height of the uterine fundus.
Ultrasound calculations are based on the fact that during certain periods of development the fetus has different physiological characteristics. The duration of pregnancy is determined by the device based on the actual parameters of the fetus and standards corresponding to different periods of gestation.
The hCG test detects the level of the hormone in the blood. From the moment of fertilization, hCG constantly increases until the 10th week. Further, its concentration decreases and increases again after 34 weeks of pregnancy. For example, at 5-6 weeks the hCG level reaches 31-128 thousand honey/ml, and at 7-8 weeks – 209-291 thousand honey/ml.
The size of the uterus during a gynecological examination also allows the gynecologist to determine the gestational age. The method is based on the fact that, starting from the moment of fertilization, the uterus constantly increases in size. During pregnancy at 4 weeks it takes on the size of a chicken egg, at 8 weeks it resembles a goose egg. However, this method is reliable only in the early stages of pregnancy.
Measuring the height of the uterine fundus allows you to determine the obstetric week of pregnancy after the uterus begins to stand out a little (from 8 to 12 weeks). To do this, the doctor feels the woman’s abdomen to determine where the fundus of the uterus is (the place where the hard part transitions to the soft part). Next, using a centimeter tape, measure the distance from the found place to the highest point of the abdomen.
The gestational age corresponds to a figure approximately equal to the resulting distance (plus or minus 3 cm). However, there is a possibility of misidentification in the case of intrauterine growth retardation.
Useful video about gestational age and PPD
List of sources:
- Radzinsky V.E. Obstetrics: a textbook for obstetric departments of secondary specialized medical educational institutions. – M.: GEOTARMEDIA, – 2008. – 900 p.
- Ailomazyan E.K. Obstetrics: a textbook for honey. universities – 3rd ed., rev. – SPb.: SpetsLit, – 2009. – 496 p.
- Kulakov V.I., Manukhina I.B. and others. Gynecology: national guidelines. – M.: GEOTAR-Media, – 2011. – 1088 p.
- Likhachev V.K. Practical gynecology: a guide for doctors. – M.: Medical Information Agency LLC, 2007. – 664 p.
By
Damira Abdel-Kader
Family doctor
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Calendars and counters online
A modern woman does not need to spend a long time and painfully with a pencil and a calendar, adding or subtracting months to calculate the expected date of birth. Today there are a lot of different calendars and counters. In them it is enough to enter the date of the last menstruation and the duration of your menstrual cycle , and here is the result - the current due date, the expected day of birth, and the serial number of the month, trimester, and even the probability of the birth of a boy or girl (everything except this point is true , the last point is for fun, comes true with a 50/50 probability).
Some counters are equipped with detailed functions: they not only count everything, but also provide detailed information about how the baby is developing week by week, what his height and weight are, and development norms. The meters are free .
