By week

Pregnancy Calendar


During pregnancy, significant changes occur in the mother’s body, which are necessary for the development of the pregnancy itself, its correct course and successful delivery. The reproductive organs of the mother develop an embryo from a fertilized egg, which later, from week 11, is called a fetus.

According to current WHO (World Health Organization) recommendations, the normal duration of a physiologically normal pregnancy is considered to be between 38 and 42 obstetric weeks. Delivery during this period is called term, i.e. on time. Delivery before 38 weeks is called premature, and after 42 weeks is called late.

What are some pregnancy calendars

There are two types of pregnancy calendars used in obstetrics.

  • Embryonic pregnancy calendar – corresponds to the weeks of development of the embryo/fetus and starts counting from the moment of fertilization. It is more accurate and specialized because it reflects directly the development of the fetus, but it is not always possible to determine the exact time of fertilization. It is believed that fertilization takes up to 2 weeks on average
  • Obstetric calendar of pregnancy – is the most convenient, widespread and often used in everyday obstetric practice, in fact, “standard”. It counts from the first day of the last menstruation, totaling up to 42 weeks.
    In order to designate the main stages in the formation of the fetus, as well as for the possibility of correct assessment and prediction of certain events, the standard division of pregnancy into trimesters, which include weeks, is used to accurately account for the term.

A week is a short period of 7 calendar days, which is considered to be conditionally minimal, and when comparing two adjacent to each other, sufficient reliable differences are recorded. As mentioned above, the duration of pregnancy is 38-42 obstetric weeks.

Trimester is a longer stage in the course of pregnancy, during which there are fundamental different processes of embryo/fetus formation.

In everyday practice, it is optimal to indicate trimesters of the same duration, although they do not coincide in terms of the main processes of fetal development.

  • I trimester – embryonic (up to 10 obstetric weeks). It is characterized by the development of the embryo from a fertilized egg. During this time, all the most important tabs of tissues, organ systems and organs themselves are formed, as well as the formation of extraembryonic structures, in particular, the formation of the placenta.
  • II trimester – fetal (from 11 to 26 weeks). The main characteristic is full differentiation of tissues, completion of the formation of all organs, organ systems. Becoming its own system of regulation of internal processes.
  • III trimester – prenatal (from 27 weeks to delivery/40-42 weeks). Since all tissues, organs, structures are formed, there is their further maturation, full and final adjustment of functions, including regulatory processes. The fetus gradually begins to prepare for the upcoming process of labor and extrauterine existence.

Division into trimesters and weeks in pregnancy is of great importance, not only and not so much in predicting the timing of the onset of labor, but because of the fact that unfavorable factors can lead to different manifestations, and moreover at different stages – the sensitivity of the embryo / fetus to them different.

The table below summarizes the main stages that occur in the developing embryo/fetus, the influence of negative factors and the significance of the resulting chromosomal abnormalities. The information is presented in the abstract, in a greatly simplified version

Obstetrical week of pregnancy Main events Influence of unfavorable factors The impact of chromosomal abnormalities
1 Preparing a woman’s body for the likelihood of pregnancy. In the first trimester, the influence of unfavorable factors is a threat to the course of pregnancy or causes significant developmental abnormalities, most often incompatible with life. Many chromosomal abnormalities lead to spontaneous termination of pregnancy, or the formation of marked developmental abnormalities characteristic of such syndromes as Down, Edwards, Turner, Klinefelter and others. Often observed the development of mosaic forms of chromosomal diseases.
3 Fertilization and implantation of the embryo.
4 Laying of germ sheets. Beginning of hCG synthesis.
5 Laying of organ systems – nervous, circulatory.
6 Formation of limb rudiments.
7 Development of the future brain.
8 Development of facial organs, fingers and hands.
9 Formation and straightening of the spine.
10 Completion of the formation of extraembryonic structures.
11 The formation of sexual characteristics. In the second trimester, unfavorable factors carry less threat for pregnancy termination, but there is a risk of systemic or organ pathology that is not associated with genomic abnormalities. These are mostly medium-sized disorders of organs and systems. Chromosomal abnormalities can be clinically detected by screening. Invasive and non-invasive gene techniques are used for complete confirmation. Abnormal development of organs and systems in the picture of the existing pathology continues. In the early stages, it is still possible to form soft “mosaic” forms. Developmental anomalies associated with gene abnormalities are formed more often.
12 All systems are formed.
13 Completion of placenta formation.
14 Completion of skin formation.
15 Abrupt increase in heart capacity.
16 Emergence of facial expressions.
17 Storage of brown fat.
18 Completion of the formation of ear structures.
19 Limbs are formed.
20 Active movements.
21 Formation of the sense of taste.
22 More organized motor activity.
23 Laying down subcutaneous fat.
24 Increased fetal size.
25 Completion of joint formation.
26 Opening of eyes, further growth.
27 All the rest of the time the fetus gradually and systematically develops, and all organs and systems mature and adjust their own work, there is an increase in weight due to fat deposits. Improves neuro-humoral regulation. The fetus is preparing for the upcoming birth and life outside the womb. At this stage, unfavorable factors may be reflected in the form of delays in the rate of intrauterine development, may increase the risk of premature birth, and may also affect the correctness of the psychophysical development of the born child. At this stage, confirmation of chromosomal abnormalities has already been obtained. There is clear visualization even with standard sonography. Even with chromosomal abnormalities, the fetus continues to develop and is also preparing for the upcoming birth.

The pregnancy calendar is an excellent tool in everyday obstetric practice, not to predict the due date of delivery, but to pay particularly close attention to certain moments in the development of the embryo/fetus and to minimize the likelihood of adverse external or internal influences and, if possible, to minimize the consequences of these influences.

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