Obstetrics and Gynecology - A.M. Gromova 2000

Diagnosis of Pregnancy. Antenatal Care
Determination of Gestational Age

Pregnancy in women lasts an average of 280 days, or 40 weeks. Various indicators are used to determine the gestational age:

- date of the last menstruation;

- date of ovulation;

- data from the initial prenatal visit;

- first fetal movement (quickening);

- objective data on the size of the Uterus and fetus.

Each of these indicators has varying significance, and only their combined evaluation allows for an accurate determination of gestational age.

1. Gestational age by menstruation is calculated based on the time elapsed since the first day of the last menstrual period. This is estimated approximately, as pregnancy cannot occur During the first week of the cycle since egg maturation has not yet taken place.

2. A more accurate METHOD FOR DETERMINING gestational age is based on ovulation—for this, 10-14 days required for egg maturation and ovulation are added to the first day of the last menstrual period, after which the gestational age is calculated.

3. One of the most important indicators for establishing gestational age is the data from the first antenatal visit, provided it occurs in early pregnancy (up to 12 weeks). Through a vaginal examination in early pregnancy, the size of the uterus allows for a fairly accurate determination of gestational age—with a margin of error of 1-2 weeks—from which the due date and current age of the pregnancy can be calculated.

4. When determining gestational age, the date of the first fetal movement is taken into account. Women pregnant for the first time usually feel this at 20 weeks, while multiparous women feel it earlier—at 18 weeks. Fetal movement is a rather subjective sign; pregnant women frequently mistake intestinal peristalsis for fetal movement, so this indicator should be considered auxiliary.

5. Data from objective examinations—determining uterine size, abdominal girth, and fetal dimensions—are of great importance for establishing gestational age.

In the first months, gestational age is determined by the size of the uterus assessed during a vaginal examination. The earliest time pregnancy can be diagnosed based on uterine dimensions is 5 weeks:

- II obstetric months (8 weeks): the size of the uterus corresponds to a goose egg;

- III obstetric months (12 weeks): it reaches the size of a newborn's HEAD, and its fundus is at the level of the upper edge of the Pubic Symphysis.

Starting from the IV month of pregnancy, the uterine fundus can be palpated through the abdominal wall, and the gestational age is estimated by the height of the fundus above the pubic bone (Fig. 11).

- IV obstetric month (16 weeks): the uterine fundus is located four finger-widths above the symphysis (6-7 cm).

- V obstetric month (20 weeks): the uterine fundus is two finger-widths below the navel (12-14 cm).

- VI obstetric month (24 weeks): the uterine fundus is at the level of the navel (20-24 cm).

- VII obstetric month (28 weeks): the uterine fundus is located 2-3 fingers above the navel (24-28 cm).

- VIII obstetric month (32 weeks): the uterine fundus is positioned midway between the navel and the xiphoid process (28-30 cm). The navel begins to flatten. Abdominal girth at the navel level is 80-85 cm.

- IX obstetric month (36 weeks): the uterine fundus rises to the xiphoid process (32-34 cm)—this is the highest level of the pregnant uterine fundus. The average abdominal girth is 90 cm. The navel is smoothed out.

- X obstetric months (40 weeks): the uterine fundus drops to the level observed at the VIII obstetric month, i.e., midway between the navel and the xiphoid process (28-30 cm). The navel protrudes. Abdominal girth is 95-98 cm, the fetal head descends, and in primigravidae, it engages with the pelvic inlet or rests as a small segment in the pelvic inlet.

While the fundal height is at the same level during the VII and X months of pregnancy, they can be differentiated by abdominal girth (80-85 cm at 8 months vs. 95-98 cm at 10 months), THE POSITION OF the fetal head (high and balloting above the pelvic inlet at 8 months vs. descended and fixed in the pelvic inlet for primigravidae at 10 months), and the state of the navel (flattened at 8 months vs. protruding at 10 months).