Calculation of Gestational Age from Last Menstrual Period - An Overview

Introduction:

Calculating the gestational age is crucial in pregnancy and preventing postdate pregnancy complications. Gestational age is a critical piece of data utilized by physicians to determine when the screening tests will be conducted and to assess the mother and the fetus throughout gestation. The physicians can calculate the gestational age at any time during the pregnancy. There are several methods of calculating the gestational age, each of which requires diverse equipment and skills. The correct estimation of the gestational age facilitates better management of pregnant women. In earlier days, physicians depended on detailed medical history and physical examination findings due to the lack of technology. The last menstrual period date is of greater significance in determining the gestational age. Nowadays, the ultrasonography procedure is a non-invasive method that is helpful in the accurate calculation of gestational age.

What Are the Methods for Determining Gestational Age?

Naegele's Rule:

• According to this rule, obtain the date of the last menstrual period by taking a thorough medical history. Add one year and seven days to the date and then subtract three months. This will approximately determine the delivery date. The last menstrual period date is the approximate start date of the fetus's age.
• It is based on the calculation that the women have a twenty-eight-day menstrual cycle.
• The variation can occur slightly because the gestation is longer for first-time mothers.
• The rule must be revised as only some women know their last menstrual period.
• Some women have twenty-one days or thirty-five days of their menstrual period.

Uterus Size:

• The uterus is a soft pelvic organ. The fertilization of the egg and sperm occurs, and the uterus increases in size to accommodate the growing fetus. The pubic symphysis is a joint between the left and right pelvic bone. At twelve weeks of gestation, the uterus is palpable above the pubic symphysis. At the sixteen weeks of pregnancy, the uterus is noticeable between the navel and the pubic symphysis. At twenty weeks of gestation, the uterus is at the navel level. This is a method of estimating the gestational age.

Determining the Conception Date:

• The human egg can fertilize with sperm for only twelve to twenty hours after ovulation (discharge of an egg from the ovary). The date of ovulation is the date of conception.
• There are more accurate methods of calculation than this method.

Ultrasonographic Methods:

These methods use ultrasound waves to produce images of internal organs.

1. Gestational Mean Sac Diameter:

• The gestational sac is a fluid structure that surrounds the fetus during the earlier stage of pregnancy.
• This is used in the first trimester.
• The transabdominal ultrasound helps in the identification of gestational sac by five weeks.
• The gestational sac is the enlarged cavity of fluid that surrounds the fetus.
• The first image in the longitudinal plane obtains long axis, anteroposterior, and width measurements.
• These three measurements are averaged to obtain the gestational sac mean diameter.
• The accuracy of gestational mean diameter as a measure of gestational age is plus or minus one week.
• The mathematical formula to calculate the gestational age is 28 days (four weeks) plus (millimeters into days) which is one millimeter per day.
• This allows us to detect if the pregnancy will be normal or if a gestational miscarriage has occurred.

2. Crown-Rump Length:

• This involves the measurement of fetal size from the top of the head to the end of the buttocks(rump).
• The measurement of the embryo is possible after six to seven weeks of gestation.
• The parameter becomes less accurate for determining the gestational age due to normal embryo development and difference in the anatomic position of the fetus.

3. Bi-parietal Diameter:

• The diameter between two sides of the fetal head. The diameter gives the gestational age.
• The measurement is done after thirteen weeks.
• Between the twelve to twenty-six weeks of gestation, the bi-parietal diameter is accurate to within plus or minus ten to eleven days.
• The calculation of gestational age by this method after six months is inaccurate; the difference is plus or minus three weeks.
• When the fetal head is oval, this method is accurate.
• When the fetal head is brachycephalic (unusually round) or dolichocephalic (unusually elongated), the bi-parietal diameter will either overestimate or underestimate the gestation age accordingly.

4. Head Circumference:

• In this method, the circumference of the fetal head is measured.
• The calculation of gestation is usually done after thirteen weeks of pregnancy.

5. Abdominal Circumference:

• The abdominal circumference is measured at the level of the fetus's liver.
• The most crucial measurement to be made in late pregnancy.
• This reflects the fetal size and weight.
• Accurate method for gestational age estimation.

6. Femur Length:

• The largest of the long bones and easiest to image.
• The femur can be adequately visualized from the fourteen weeks of gestation until delivery.
• Measurement is along the long axis of the bone.
• The length can be accurately used to predict the gestational age after fourteen weeks until delivery.

Measuring With a Soft Tape:

• The uterus consists of three parts that are fundus, the upper part, the body, and the cervix, the opening to the vagina. The tape measure is placed on the pubic symphysis (a joint that connects the two pelvic bones) and at the fundus of the uterus at the other.
• When the fundus is above the umbilicus (navel), the number of centimeters is said to correlate with the number of weeks. This gives the gestational age in weeks.

What Are the Advantages of Ultrasonographic Techniques?

• Ultrasound utilizes ultrasound waves to develop images of internal organs, allowing early pregnancy detection and early estimation of the delivery date (EDD).
• Early detection of pregnancy in thirty days.
• This is more accurate than the last menstrual period method.

What Are the Disadvantages of Ultrasonographic Techniques?

• In some methods, the fetus has to be in a particular position, or the head has to be of a specific shape.
• Fetal growth patterns are not considered.
• Maternal age and prior history of pregnancy are not taken into account.
• There may be errors while measuring.
• Twins or more can give a significant fundal height for the gestational age.
• Oligohydramnios is decreased amniotic fluid and decreased fundal height.
• Polyhydramnios is increased amniotic fluid around the fetus, increased fundal height, and the false detection of gestational age.

Conclusion:

The gestational age is an essential criterion for maintaining the viability of gestation. The error in calculating the gestation age can lead to prolonged pregnancy and cause complications for the mother and the fetus. Ultimately the well-being of the mother and child is essential.