You are in: eMedicine Specialties > Obstetrics and Gynecology > General Obstetrics Evaluation of GestationArticle Last Updated: Jan 12, 2007AUTHOR AND EDITOR INFORMATIONSection 1 of 8
  Author: Max Mongelli, MD, Associate Professor, Department of Obstetrics and Gynecology, University of Sydney; Senior Consultant, Department of Obstetrics and Gynecology, Nepean Hospital, New South Wales Coauthor(s): Michael Peek, PhD, Professor, Associate Dean, Faculty of Medicine, University of Sydney; Consulting Staff, Department of Obstetrics and Gynecology, Nepean Hospital; Jason Gardosi, MD, Professor, Department of Obstetrics and Gynecology, University of Birmingham; Director, West Midlands Perinatal Institute Editors: Suzanne R Trupin, MD, Clinical Professor of Obstetrics and Gynecology, University of Illinois College of Medicine-Champaign; CEO and Owner, Women's Health Practice; CEO and Owner, Hada Cosmetic Medicine and Midwest Surgical Center; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Richard S Legro, MD, Professor, Department of Obstetrics and Gynecology, Division of Reproductive Endocrinology, Pennsylvania State University College of Medicine; Consulting Staff, Milton S Hershey Medical Center; Frederick B Gaupp, MD, Consulting Staff, Department of Family Practice, Assumption Community Hospital; Lee P Shulman, MD, Professor of Obstetrics and Gynecology, Feinberg School of Medicine, Northwestern University; Chief, Division of Reproductive Genetics, Department of Obstetrics and Gynecology, Prentice Women's Hospital, Northwestern Memorial Hospital Author and Editor Disclosure Synonyms and related keywords: evaluation of gestation, gestational age, GA, conceptual age, CA, estimated date of confinement, EDC, expected delivery date, expected date of delivery, estimated due date, EDD, prenatal assessment, diagnostic evaluation of pregnancy, evaluation of gestation, pregnancy, obstetrics and gynecology, OB/GYN, OB-GYN, obstetrical evaluation, menstrual history, last menstrual period, LMP, ultrasound, ultrasonography, fetal crown-rump length, CRL, biparietal diameter, BPD, head circumference, HC, femur length, FL INTRODUCTIONSection 2 of 8
  The estimation of pregnancy dates is important for the mother, who wants to know when to expect the birth of her baby, and for her health care providers, so they may choose the junctures at which to perform various screening tests and assessments (Peek, 1994), which include (1) serum screening, (2) assessment of maturity (eg, as in threatened preterm labor), and (3) induction of labor for postdate pregnancies. The 3 basic methods used to help estimate gestational age (GA) are menstrual history, clinical examination, and ultrasonography. The first 2 are subject to considerable error and should only be used when ultrasonography facilities are not available. The date of feeling the first fetal movements (quickening) is far too unreliable to be useful. In rare cases, the date of coitus is known, and this may be useful in calculating the length of pregnancy. Terminology GA refers to the length of pregnancy after the first day of the last menstrual period (LMP) and is usually expressed in weeks and days. This is also known as menstrual age. Conceptional age (CA) is the true fetal age and refers to the length of pregnancy from the time of conception. This terminology does vary geographically and over time, and it may need clarification if not explicitly defined in relevant articles. CLINICAL METHODS OF ESTIMATING GESTATIONAL AGESection 3 of 8
Menstrual history Gestational age (GA) has traditionally been estimated from the date of the last menstrual period (LMP). That conception occurs on day 14 of the cycle is assumed. The fallacy in this assumption is that the time of ovulation in relation to the menstrual cycle varies greatly, both from cycle to cycle and individual to individual. Basing GA on the LMP tends to result in an overestimation. The 95% confidence interval of menstrual dates is -27 to +9 days (Wilcox, 1993). To further complicate matters, 10-45% of pregnant women cannot provide useful information about their LMP, and 18% of women with certain menstrual dates have significant differences between menstrual and ultrasonographic dating (Geirsson and Busby-Earle, 1991). The accuracy of menstrual history in women with a history of oligo-ovulation, such as those with polycystic ovarian syndrome, should be questioned. If pregnancy occurred while oral contraceptives were being taken, the LMP cannot be used because such measures have no bearing on ovulation. Date of pregnancy test Knowing the date of the first positive pregnancy test result allows the calculation of a minimum GA. This depends on the sensitivity of the test. For example, if the test was performed 4 weeks ago and the test is known to return positive results as early as 1 week after conception, then the minimum CA would be 5 weeks (GA, 5 + 2 = 7 wk of amenorrhea). This information can be useful in clinical practice if the test finding has been documented by a health care professional. Pregnancy following assisted reproduction techniques Not infrequently, practitioners are called upon to manage pregnancies resulting from in vitro fertilization or intrauterine insemination. The GA of pregnancies resulting from in vitro fertilization can be precisely calculated from the time of embryo replacement; however, conception may be delayed for a few days in pregnancies resulting from intrauterine insemination. In patients who have had ovulation induction, calculate GA from the day of human chorionic gonadotropin administration. Clinical examination The size of the uterus can be assessed by pelvic examination or by abdominal palpation. Size can be misleading in the presence of multiple pregnancy, uterine fibroids, or a full bladder. Tape measurement of the symphysis-fundus height may be useful up to 28-30 weeks' gestation, beyond which it becomes too inaccurate for dating. Perception of fetal movement The perception of fetal movement by the patient, often referred to as quickening, is a relatively late sign of pregnancy, usually occurring at 19-21 weeks' gestation in nulliparous women and 17-19 weeks' in multiparous women. Before the advent of pregnancy tests and ultrasonography, quickening was often the method by which a suspected pregnancy was confirmed; however, at present, it has little diagnostic value. ESTIMATING THE DELIVERY DATESection 4 of 8
The expected date of delivery (EDD) is one of the earliest pieces of information a pregnant woman requests once pregnancy is confirmed. In order to calculate this, the practitioner must know the median length of normal pregnancy and the last menstrual period (LMP) or ultrasonographic estimation of gestational age (GA). Pregnant women should be counseled that only 4% of all babies are born precisely on the estimated date of confinement. Failure to appreciate this may lead to unnecessary maternal anxiety if a pregnancy progresses beyond the EDD. Therefore, giving a range for the likely date of birth (eg, estimated date of confinement ± 2 wk) is more useful. The median length of human pregnancy is 280 days of amenorrhea (from the first day of the LMP) or a CA of 266 days (280 - 14). Infants born before 37 completed weeks' gestation are deemed preterm, whereas those born after 42 weeks' are considered postterm. In normal pregnancies, the length of gestation is not affected by maternal characteristics. The Nägele rule and the obstetric wheel In women with regular cycles and a certain LMP, the EDD is calculated by adding 7 days to the first day of the LMP and adding 9 months (Nägele, 1836). For example, with an LMP beginning on June 15, the EDD will be March 22 of the following year. Most antenatal clinics have obstetric wheels. These consist of an outer wheel that has markings for the calendar and an inner, sliding wheel with weeks and days of gestation. They facilitate the estimation of GA and the calculation of the EDD. The quality of these wheels varies, but in general, the larger wheels yield better results. Dates calculated on the basis of the LMP are often inaccurate because the time of ovulation can be extremely variable in relation to the occurrence of menses. Furthermore, cycle lengths among women vary greatly, as they do for individual women, changing from cycle to cycle, with a standard deviation of plus or minus 2.5 days. The tendency is toward longer anovulatory cycles, and in such women, the error in GA estimation can be much greater (Geirsson and Busby-Earle, 1991). ULTRASONOGRAPHIC ASSESSMENT OF GESTATIONAL AGESection 5 of 8
The introduction of obstetric ultrasonography in the early 1970s led to a marked improvement in the evaluation of fetal and placental anatomy, as well as fetal growth. Now, it is by far the most accurate technique for estimating gestational age (GA). Most pregnant women have a first trimester scan, followed by a detailed scan for anomalies in the second trimester. Other documented benefits of obstetric ultrasonography include a reduction in perinatal mortality, the detection of multiple pregnancies and fetal abnormalities, and the identification of placenta previa. Ultrasonography in the first trimester GA in the first trimester is usually calculated from the fetal crown-rump length (CRL). This is the longest demonstrable length of the embryo or fetus, excluding the limbs and the yolk sac (see Image 1). The correlation between CRL and GA is excellent until approximately 12 weeks' amenorrhea. No sex or race differences are appreciable. The GA estimate has a 95% confidence interval of plus or minus 6 days. The following formula (Westerway, 2000) allows the estimation of GA (weeks) from the CRL (mm): GA = –0.0007 (CRL)2 + 0.1584 (CRL) + 5.2876 Ultrasonography in the second trimester Fetal biometry in the second trimester can yield acceptably accurate estimates of GA from 12 to approximately 22 weeks of amenorrhea (Westerway, 2000). Recent work has shown that the accuracy of ultrasonographic biometry at 12-14 weeks' gestation is at least as good as biometry performed after 14 weeks (Sladkevicious, 2005; Saltvedt, 2004). The best parameters are the biparietal diameter (BPD) and the head circumference (HC), which are virtually linearly related to GA (see Image 2). The femur length (FL) can also be used and is nearly as accurate as head measurements (Mongelli, 2003) (see Image 3). Racial differences in FL are significant, but differences in HC are not. GA estimates by the BPD or HC have a 95% confidence interval of plus or minus 8 days. The following formula (Persson, 1986) allows estimation of GA (days) from the BPD (mm): GA = 39.1 + 2.1 (BPD) Ultrasonography in the third trimester Fetal biometry in the third trimester is subject to much greater individual size variations than in the second trimester. Its accuracy for GA assignment is reduced considerably, and estimates may have confidence intervals of plus or minus 3 weeks (Hadlock, 1984). More recent work with pregnancies resulting from in vitro fertilization suggests that third trimester scans are considerably more accurate, with random errors (1 SD) of 8-9 days. This is equivalent to confidence intervals of about plus or minus 2 weeks (Mongelli, 2005). The table in Image 4 illustrates the 95% confidence limits for GA estimated from the BPD in the third trimester. COMBINING MENSTRUAL AND ULTRASONOGRAPHIC DATESSection 6 of 8
In many ultrasound departments, a common practice is to combine the last menstrual period (LMP) with ultrasonographic dates in what is known as the 10-day rule or 7-day rule. For example, with the 10-day rule, if LMP dates and ultrasonographic dates are in agreement within 10 days, LMP dates are accepted. On the other hand, if the discrepancy exceeds 10 days, ultrasonographic dates are used. The rationale for using these rules is to exclude large errors from incorrect menstrual dates. The implicit assumption of this method is that menstrual dating is preferable to ultrasonographic dating. Detailed analysis from large databases has not shown any advantage in using these rules (Mongelli, 1996). Unless the fetus is thought to be anatomically abnormal, ultrasonographic dates may be used for all pregnancies if a scan is available in the first half of pregnancy. However, combining menstrual dates with ultrasonographic dates is still widely practiced in the United States and in Australia. Gestational age calculators Computer software is now available for accurate determination of GA from either ultrasonographic biometry or menstrual dates, and results are more accurate than those obtained with obstetric wheels. Such software is accessible online through the Gestation Network and can be downloaded free of charge (see Pregnancy Dating - Gestational Age Calculator). MULTIMEDIASection 7 of 8
REFERENCESSection 8 of 8
Evaluation of Gestation excerpt Article Last Updated: Jan 12, 2007 | |||||||||||||||||||||||||||||||
