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due date calculator
Determining the duration of pregnancy and the expected date of birth is very important for both the woman and the doctor leading her. Many laboratory and instrumental studies are prescribed precisely at the time when it is easiest to identify the pathology in time. Comparison of fetal parameters with the average statistical indicators of each period of pregnancy helps to identify and correct underdevelopment. Clarity in determining the date of birth of a child allows you to carefully prepare for this event and open a sick leave in time so that the woman has time to gain strength before childbirth.
There are several methods to help calculate the required indicators. An obstetrician-gynecologist uses them in combination, since almost every method has an allowable error associated with subjective reasons: the individual characteristics of a woman, elementary forgetfulness – it is difficult to remember the date of the last menstruation. The term of the expected birth can be determined quite approximately, since a full-term pregnancy lasts from 38 to 42 weeks, and this is normal from the point of view of physiology.
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Formula Negele
The method of calculating the due date using this formula is great for women with a regular standard cycle lasting 28 days. Negele’s formula is simple – to determine the desired date, you need to subtract 3 months from the first day of the last menstruation, and then add 7 days to the resulting figure. If the menstrual cycle was constant, but lasted more or less than the standard one, the difference between your own and the standard cycle should be added to the date obtained, or the same number of days should be subtracted.
You can calculate even easier – add 280 days (or 40 weeks) to the start date of the last menstruation.
How to calculate due date based on ovulation date?
If a woman in the month preceding conception determined the onset of ovulation by basal temperature, this will help to accurately determine the gestational age. Conception occurs on the day of ovulation, when the egg ready for fertilization leaves the ovary. If sexual intercourse took place 1-2 days before ovulation, spermatozoa remain ready for fertilization all this time, that is, conception can occur a short time after sexual contact. If you add 38 weeks, or 266 days, to the date of ovulation, you can find out the estimated date of birth of the child.
If a woman does not know when she ovulated, but has a stable 28-day menstrual cycle, it is assumed that conception occurred in the middle of the cycle, that is, on day 14. With an irregular cycle, this method has an error.
We determine the date by the movement of the child
The date when a woman first felt the baby move is very important for correcting a comprehensive calculation of the term of the expected birth. Although the fetus begins to move as early as the twelfth week, for the first time most women feel these gentle movements in the middle of the gestational period, that is, in the twentieth week. However, this rule does not apply to everyone – during the second and subsequent pregnancies, a woman may feel the first movements 2 weeks earlier.
A primiparous woman of a fragile physique also has such an opportunity, while a pregnant woman with a solid build feels movements later. These sensations can be called subjective, because they depend on the woman’s sensitivity to what is happening in her body. The thickness of the myometrium also affects the severity of the movements of the child.
All these nuances mean that the above method of determining the date of birth cannot be considered the main one, it is used only in combination with other methods.
How do obstetricians-gynecologists calculate the gestational age?
When calculating the gestational age, the doctor usually relies on the start date of the last menstruation, however, the main calculations are carried out taking into account a complex of various criteria.
According to the size of the uterus
In the early stages of pregnancy, when individual differences in fetal size are not yet apparent, it is possible to determine the date of delivery using this technique. For a period of 4 weeks, the size of the uterus is compared with a chicken egg, for a period of 8 weeks – with a goose.
From the twelfth week, this method is already difficult to apply, since in the future the parameters of the uterus and fetus differ in different women with the same period.
By ultrasound
The method of determining the date of birth according to ultrasound scanning is valid at the beginning of the gestational period, more precisely, until the third month. After twelve weeks, it is impossible to accurately determine the date of birth based on ultrasound data due to individual differences in fetal parameters in women of different constitutions.
According to the height of the fundus of the uterus
This method has been used by the obstetrician-gynecologist since the beginning of the second trimester. The longer the fetus develops, the higher the bottom of the organ becomes, which rises beyond the small pelvis.
The height of the fundus of the uterus is determined when the woman is lying on her back. The doctor carefully palpates the abdomen and measures the distance from the pubic symphysis to the fundus of the uterus. This indicator increases evenly throughout the gestational period, and immediately before childbirth, the uterus descends.
When using this technique, an error of 2-3 cm is laid. It can be caused by polyhydramnios, pregnancy with twins or triplets, the age of the woman, and the size of the fetus. To clarify the indicators, the waist circumference of the pregnant woman and the parameters of the fetus are taken into account.
Calculation of the height of the fundus of the uterus by week:
8-9 weeks – height 8-9 cm, the uterus is still located within the small pelvis;
10-13 weeks – height 10-11 cm, the uterus increases, the placenta develops intensively, the fetal cardiovascular system begins to work;
16-17 weeks – height 14-18 cm, the uterus is located between the navel and the pubic joint, the fetus looks like a little man, all of whose organs are already laid;
18-19 weeks – height 18-19 cm, the unborn child develops arms and legs with fingers, the cerebellum, the immune system, the placenta fully assumes the responsibility for ensuring the functioning of the fetus;
20 weeks – the height of the uterus in centimeters corresponds to the gestational age in weeks;
21 weeks – height 21 cm, from now on the height of the uterus will increase by 1 cm weekly, now it is 2 fingers below the navel.
22-24 weeks – height 22-24 cm, the bottom of the uterus at the level of the navel, easily determined by palpation, the body weight of the fetus is 0,6-0,7 kg;
25-27 weeks – height 25-28 cm;
28-30 weeks – height 28-31 cm;
32 weeks – height 32 cm, the upper edge of the uterus is located between the xiphoid process of the sternum and the navel;
36 weeks – height 36-37 cm, the uterus is palpable at the line of connection of the costal arches;
39 weeks – height 36-38 cm, the uterus begins to descend, the weight of the fetus exceeds 2 kg;
40 weeks – the height of the uterus decreases to 32 cm, it is again between the navel and the ribs, the child is ready for delivery.
Head size and fetus length
To calculate the gestational age, the following methods are used:
Method Jordan. The term is calculated according to the formula X=L+C, where X is the term in weeks, L is the length of the body of the fetus (cm), C is the diameter of its head (cm).
Skulsky method. Here the formula is: X = (L x 2) – 5/5, where L is the length of the fetal body (cm), the number 5 in the numerator of the fraction is the thickness of the myometrium, the number 5 in its denominator is a special coefficient.
Any calculations are only an approximate guideline. According to medical statistics, only 10-17% of babies are born on time, the rest are born earlier or later than the expected date. This circumstance is influenced by many factors: heredity, pregnancy pathologies, stressful situations and other reasons.