Rule of eights pregnancy dating

Methods for Estimating the Due Date - ACOG

rule of eights pregnancy dating

As the end point date in pregnancy, the estimated date of delivery provides Naegele's rule, pregnancy dating, ultrasound in pregnancy, Rule of Eights. As the end point date in pregnancy, the estimated date of delivery provides called the rule of eights can be used to determine a final estimated date of delivery. Gestational age is a measure of the age of a pregnancy which is taken from the . A Simple Solution to Dating Discrepancies: The Rule of Eights Hunter, L. A.

Issues in pregnancy dating: revisiting the evidence.

Because this practice assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle, this practice does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation. It has been reported that approximately one half of women accurately recall their LMP 2—4. Accurate determination of gestational age can positively affect pregnancy outcomes. For instance, one study found a reduction in the need for postterm inductions in a group of women randomized to receive routine first-trimester ultrasonography compared with women who received only second-trimester ultrasonography 5.

A Cochrane review concluded that ultrasonography can reduce the need for postterm induction and lead to earlier detection of multiple gestations 6. Because decisions to change the EDD significantly affect pregnancy management, their implications should be discussed with patients and recorded in the medical record.

Measurements of the CRL are more accurate the earlier in the first trimester that ultrasonography is performed 11, 15— The measurement used for dating should be the mean of three discrete CRL measurements when possible and should be obtained in a true midsagittal plane, with the genital tubercle and fetal spine longitudinally in view and the maximum length from cranium to caudal rump measured as a straight line 8, Mean sac diameter measurements are not recommended for estimating the due date.

Dating changes for smaller discrepancies are appropriate based on how early in the first trimester the ultrasound examination was performed and clinical assessment of the reliability of the LMP date Table 1. For example, for a day-5 embryo, the EDD would be days from the embryo replacement date.

Gestational age

Likewise, the EDD for a day-3 embryo would be days from the embryo replacement date. Clinical Considerations in the Second Trimester Using a single ultrasound examination in the second trimester to assist in determining the gestational age enables simultaneous fetal anatomic evaluation. With rare exception, if a first-trimester ultrasound examination was performed, especially one consistent with LMP dating, gestational age should not be adjusted based on a second-trimester ultrasound examination.

Ultrasonography dating in the second trimester typically is based on regression formulas that incorporate variables such as the biparietal diameter and head circumference measured in transverse section of the head at the level of the thalami and cavum septi pellucidi; the cerebellar hemispheres should not be visible in this scanning plane the femur length measured with full length of the bone perpendicular to the ultrasound beam, excluding the distal femoral epiphysis the abdominal circumference measured in symmetrical, transverse round section at the skin line, with visualization of the vertebrae and in a plane with visualization of the stomach, umbilical vein, and portal sinus 8 Other biometric variables, such as additional long bones and the transverse cerebellar diameter, also can play a role.

Date changes for smaller discrepancies 10—14 days are appropriate based on how early in this second-trimester range the ultrasound examination was performed and on clinician assessment of LMP reliability. Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; therefore, decisions need to be guided by careful consideration of the entire clinical picture and may require close surveillance, including repeat ultrasonography, to ensure appropriate interval growth.

The best available data support adjusting the EDD of a pregnancy if the first ultrasonography in the pregnancy is performed in the third trimester and suggests a discrepancy in gestational dating of more than 21 days. Conclusion Accurate dating of pregnancy is important to improve outcomes and is a research and public health imperative.

As soon as data from the LMP, the first accurate ultrasound examination, or both are obtained, the gestational age and the EDD should be determined, discussed with the patient, and documented clearly in the medical record.

rule of eights pregnancy dating

The American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal—Fetal Medicine recognize the advantages of a single dating paradigm being used within and between institutions that provide obstetric care. Table 1 provides guidelines for estimating the due date based on ultrasonography and the LMP in pregnancy, and provides single-point cutoffs and ranges based on available evidence and expert opinion.

Fetal Imaging Workshop Invited Participants.

rule of eights pregnancy dating

A comparison of recalled date of last menstrual period with prospectively recorded dates. J Womens Health Larchmt ; Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. Am J Obstet Gynecol ; Last menstrual period versus ultrasound for pregnancy dating. Int J Gynaecol Obstet ; First trimester ultrasound screening is effective in reducing postterm labor induction rates: Ultrasound for fetal assessment in early pregnancy.

Cochrane Database of Systematic ReviewsIssue 7. Predicting delivery date by ultrasound and last menstrual period in early gestation. New charts for ultrasound dating of pregnancy and assessment of fetal growth: Ultrasound Obstet Gynecol ; First- and second-trimester ultrasound assessment of gestational age.

Beyond 18 weeks of gestation, a full fetal anatomic survey is possible. Indications For appropriate management of any pregnancy, practitioners must establish gestational age. Any pregnancy that does not meet this criterion should be considered suboptimally dated.

Contraindications Ultrasound has been used in obstetrics for over 50 years and is considered safe. Ultrasound does have effects on tissues due to energy used to obtain images. For this reason, only use ultrasound when it is clinically indicated and for the shortest amount of time possible. Equipment As previously described, there are multiple methods for establishing an estimated delivery date.

When using the patient's last menstrual period to establish pregnancy dating, Naegele rule requires the use of a calendar. A physical exam is an alternate method for establishing gestational age, which requires the use of an appropriately suited exam table with stirrups for a reliable pelvic exam.

For fundal height measurement, a non-elastic centimeter tape measure is needed to measure the pubic symphysis to fundus measurement. Ultrasound is the most reliable method to establish pregnancy dating, particularly first-trimester ultrasound.


Transvaginal ultrasound requires the use of a probe cover, and latex or nonlatex may be used. Beyond eight weeks, transabdominal ultrasound is typically satisfactory for evaluation of pregnancy. Abdominal obesity or a retroverted uterus may cause difficulty during transabdominal approach. A gel warmer may be used for patient comfort, but caution should be used if the warm gel is used transvaginally to ensure the gel is not too hot. In certain circumstances, an ultrasound technician will be the first person to evaluate a pregnancy using ultrasound.

Preparation Patient preparation before ultrasound varies depending on which approach is used. For transabdominal ultrasound, a full bladder is helpful but not required. Full bladder acts as an acoustic window and improves image details.

Pregnancy, Dating - StatPearls - NCBI Bookshelf

For a transvaginal ultrasound, a full bladder can displace the uterus posteriorly and out of the field of view of the transvaginal ultrasound probe. For this reason, it is recommended to perform transvaginal ultrasound with an empty bladder.

A recommended approach would be to perform transabdominal ultrasound with a full bladder, if the transvaginal approach is needed, have the patient empty her bladder before a transvaginal ultrasound. Ultrasound should be performed on a bed or stretcher which is equipped for performing pelvic exams with foot stirrups and adjustable lower half of table.

  • Women's Health Care Physicians
  • Gestational age

Abdominal ultrasound approach may be performed in the supine position. The transvaginal approach is best carried out with the patient in the lithotomy position, with the patient's buttocks at the end of the table allowing for a complete range of motion with the transvaginal ultrasound probe.

One method of estimating the delivery date in this instance is by using Naegele Rule. This technique assumes that the patient has a normal day menstrual cycle and ovulates on day 14 of that cycle. If the patient is unsure of her last menstrual period or has irregular menstrual cycles, the accuracy of this method decreases.

In many instances, a patient will not remember the first day of her last menstrual cycle or will be unsure of the exact date on which her cycle started. In this circumstance, a thorough physical examination will allow a preliminary estimation of the gestational age until ultrasound can be performed. Obviously, there are wide variations in uterine size at any given age due to gynecologic pathology e. Fruit sizes are extremely variable, so this model is widely inaccurate in certain circumstances.

The five-week pregnant uterus is approximately the size of a small, unripe pear. The six-week pregnant uterus feels like a small orange. The eight-week pregnant uterus feels like a large orange. The week pregnant uterus feels like a grapefruit. Many obstetricians develop a feel for uterine size through the clinical experience of performing thousands of bimanual exams and have their comparisons for estimating gestational age in this manner.

This method can be performed before ultrasound examination to determine the urgency for which dating ultrasound should be performed and ensure the patient has optimal pregnancy dating. Fundal height measurement is another physical exam parameter that can be used to estimate gestational age.

Gestational age | Revolvy

The distance from the uterine fundus to the pubic symphysis defines fundal height measurement. Measurement should be performed using a non-elastic tape measure, and the patient should have an empty bladder. The most common use for fundal height measurement is to record the trend to assess appropriate fetal growth and to screen for fetal growth restriction.

The usefulness of fundal height measurement in any circumstance has varied widely across the literature but can be helpful in resource-poor areas for an estimation of gestational age. The assumption that is made with fundal height measurement is that each 1 cm increment in fundal height corresponds to one week of gestation. Fundal height measurement is most useful after 20 weeks when the uterine fundus is usually at the level of the umbilicus.

A difference of two to three weeks or two to three centimeters in either direction is within the normal range. For example, a patient whose fundal height is 25 cm can have a gestational age range from 23 to 27 weeks. The assumption for this patient will be that she is approximately 25 weeks of gestation. This measurement can vary widely based on different maternal variables. Uterine fibroids, amniotic fluid abnormalities, increased maternal body mass index BMIand fetal growth abnormalities are some examples of circumstances that can alter the accuracy of fundal height measurement.

First-trimester ultrasound can be performed either transvaginally or transabdominally. Crown-rump length is measured in the first trimester, which is defined as the maximum length from cranium to caudal rump measured in a straight line. The measurement should be performed in the true midsagittal plane, with a clear view of the genital tubercle and fetal spine longitudinally.