Matching weeks to gestation and trimester

Very often, future mothers, especially in the initial periods, are confused in weeks, trimesters and months. We all know that a child is being born for 9 months, but then where does the 10th month come from, after all, there are 40 weeks in pregnancy? Detailed instructions on how to correctly calculate the terms and not to “get lost” in them are presented in this article.

For the reference period of pregnancy in the global obstetric practice introduced the concept of "obstetric week." It is in them that doctors calculate pregnancy. Therefore, an inexperienced woman who comes 7-8 days after a delay in a consultation and hears from the doctor that she has already gone. second month of pregnancy, there is bewilderment and sincere indignation - she is pregnant with nothing at all, and she is credited with too much!

From here, numerous posts and messages from the ladies "in position" are born, who are confident that "the doctors were mistaken with the deadline." Not mistaken. Just they estimated obstetric weeks, and women habitually consider everything in fact - from the “day X”, when there was sexual intercourse, which could lead to the birth of a new life.

A woman is fertile only on the day of ovulation and during the day after. But even the woman is not able to find out exactly that day sometimes. The ovum can leave the follicle a few days earlier or later, because this process can be affected by a variety of factors - stress, cold, fatigue. Thus, to establish the exact date when the baby was conceived, neither the doctor nor the woman is capable. Therefore, it was decided to measure the period in obstetric weeks. They begin to be counted from the first day of the last in this cycle of menstruation.

In this way:

• Week number one is the time when a woman has menstrual bleeding.
• The second week precedes ovulation.
• On the third, if lucky, the implantation of a fertilized oocyte into the endometrial layer of the uterus happens.
• Toward the end of the fourth week, a delay begins, which forces a woman to buy a test and establish the fact of her soon motherhood.

That is, the obstetric period is always 2 weeks longer than the actual, fetal.

We know from school that a month is 30 or 31 days. February lasts 28 or 29 days. But the obstetric month and calendar month are two different things.

Pregnancy is calculated not only in obstetric weeks, but also in obstetric months.

Therefore, the lunar month and obstetric are very similar. This means that each obstetric month lasts 28 days and not a day more:

• the first month pregnancy - 1-4 weeks inclusive (at the end a woman can only learn about her pregnancy);
• second month - 5-8 weeks inclusive;
• third month - 9-12 weeks inclusive;
• fourth month - 13-16 weeks inclusive;
• fifth month - 17-20 weeks inclusive;
• sixth month - 21-24 weeks inclusive;
• seventh month - 25-28 week inclusive.
• eighth month - 29-32 week inclusive;
• ninth month - 33-36 week inclusive;
• tenth month - 37-40 week of pregnancy.

If you look at a regular wall or desk calendar, the months of pregnancy will be distributed somewhat differently:

• the first is 1-4.5 weeks;
• the second is 4.5-9 weeks;
• the third is 9-13 weeks;
• the fourth is 14-17.5 weeks;
• fifth - 17.5-22 weeks;
• the sixth is 22-26.5 weeks;
• the seventh is 26.5-30.5 weeks;
• the eighth - 30.5-35 weeks;
• the ninth is 36-40.5 weeks.

Now you know where the tenth month of pregnancy came from and there will be nothing incomprehensible in this.

For convenience, the entire gestational period is divided into three parts. Each is called a trimester. In each trimester, three months. Thus, trimesters look like this:

• first trimester - 1-13 weeks;
• second trimester - 14-26 weeks;
• the third trimester - from week 27 to birth.

Fetal development from a single cell to a full-fledged human body, which will become a newborn, is divided into several periods. They are also closely related to obstetric terms. There are several periods:

• Preimplantation. It starts from the first day of the last menstruation (1 week) and lasts up to 21 days, when the fertilized egg can pass the way through the fallopian tube and fix in the uterine wall (that is, the implantation will take place).
• Embryonic. It starts from the moment of fertilization and lasts up to 9-10 obstetric weeks, inclusive. The most crucial period, since at this time is the laying and the formation of all internal organs and systems. The baby is called an embryo.
• Fetal (fetal). From 11 days to birth. During this time, the child grows, all organs are already formed. The baby is called the fruit.
• Perinatal (prenatal) starts from the 23rd week of pregnancy, when the birth of a child will not be considered a miscarriage, but will be classified as premature birth, lasting up to 42 weeks.
• Prolonged begins at 40 obstetric weeks if delivery has not begun.
• Late delivery - after 42 weeks of pregnancy.

Women who doubt obstetric calculations may well make their own calculations: by the day of the onset of menstruation, add the number of days in the last full cycle to the current date. Example: monthly began May 1. On the calendar August 6th. You have 9 weeks + 3 days. For the calculation you need the most ordinary calendar.

There are several ways to determine the possible date of delivery. But it should be understood that the so-called PDR (estimated date of birth) will be just approximate - no one can guarantee you that the child will decide to come to this world exactly the right number, because the births from 37 to 42 weeks are considered normal. PDR - the last day of the 40th obstetric week and no more than that approximate guide for doctors and future mothers. You can calculate your term in several ways.

If you know for sure when you had ovulation (did an appropriate test, ovulation was stimulated with hormonal drugs, etc.), you can use the following formula A + 266 = X, where A is the estimated day of conception and X is the expected day of birth. The method of obstetricians is not used due to its large error.

This is the most common method, claimed and proven by time and by many generations of obstetricians. The formula is as follows:

M + 280 = X, Where:

• M - the first day of the last menstrual period;
• X - estimated date of delivery.

The Negel formula can also be used. She looks like this:

M-3 months + 1 week. = X.

You deduct three months from the first day of the last menstruation and add seven days to get the date of birth.

Ultrasound scanning with great accuracy can help determine the duration of pregnancy. The doctor will start from the compliance of the size of the ovum (in the early stages) to the established norms. The diameter of the ovum tells you how long the baby has been developing. The method is good precisely in the early stages, since it is at the beginning of pregnancy that all children develop at almost the same rate.

Embryos are similar, which cannot be said about fruits - they begin to show individual traits (determining the excess length of the femur in a tall child may cause erroneous overpriced gestation at the actual, which will be less). Thus, bearing larger children can deliver a longer term, and mothers of tiny babies can understate it.

Earlier, when there was no ultrasound, it was this method that was widely used in antenatal clinics. Primiparas marked the time of the first movements. To this date, the doctor added exactly 20 weeks. Reversing was considered different, adding 22 weeks to the date of the baby’s first movements.

The height of the uterus floor is important. She is measured by a gynecologist at each visit by the future mother of the antenatal clinic. It usually corresponds to obstetric terms in numerical terms. But the method can be applied only when a woman has a tummy, has no use in the early stages. Moreover, with intrauterine growth retardation, improper presentation of the fetus, its transverse arrangement, the dimensions may not correspond to the term. The ratio of VSDM when carrying twins or triplets also deviates from the average norms.

Today there are a huge number of calculators and counters that will help calculate the exact date and expected date of birth. A woman will only need to specify the date of the first day of her last menstruation. The program will calculate and translate the term into obstetric terms, months and trimesters, and also report on what stage of development the baby is now at.

The features of such programs are that they do not take into account possible individual factors, for example, late ovulation, and therefore may also be mistaken, but only slightly - the error is unlikely to exceed 7-10 days. But without extra calculations, you can find out in one click how much it is - seven months of pregnancy, how much it is in days, weeks, what is the term and the period of development of the baby.