The most important step in the initial evaluation of any pregnant patient is establishing an accurate delivery date (due date). Accurate knowledge of the gestational age is important for numerous reasons. A patient’s gestational age determines the appropriate intervals for prenatal care visits, as well as the timing of certain interventions. For example, certain antenatal screening tests like the quadruple marker screen (screening test for fetal aneuploidy and open neural tube defects) must be performed with accurate knowledge of the gestational age for an accurate calculation of lab values. In patients with a history of preterm labor and delivery, screening tests and interventions early in pregnancy can be used to prevent preterm labor in any subsequent pregnancy. As pregnancy progresses, accurate and optimal pregnancy dating is important when deciding on the timing of both medically indicated and elective deliveries.
Pregnancy ultrasound involves an anatomic survey of uterus and adnexa. On the initial ultrasound, it is important to establish the location of the gestational sac, confirm the intrauterine location and document the presence or absence of yolk sac, fetal pole, and fetal number. If a fetal pole is seen, the presence of fetal heart tones should be documented. If ultrasound is performed beyond 18 weeks of gestation, a full fetal anatomic survey is possible.
For appropriate management of any pregnancy, practitioners must establish gestational age. All pregnancies should have either abdominal or transvaginal ultrasound to confirm or establish a gestational age before 22 0/7 weeks. Any pregnancy that does not meet this criterion should be considered sub-optimally dated.
When using the patient's last menstrual period to establish pregnancy dating, Naegele's rule requires the use of a calendar. Ultrasound is the most reliable method to establish pregnancy dating, particularly first-trimester ultrasound. Transvaginal ultrasound utilizes 6 MHz to 10 MHz ultrasound probe. This probe has a higher frequency than transabominal ultrasound probes, which can show intrauterine structures approximately one week earlier in gestation. Beyond eight weeks, transabdominal ultrasound is typically satisfactory for evaluation of pregnancy. Transabdominal ultrasound utilizes a curvilinear ultrasound probe with a frequency of 3 MHz to 6 MHz which provides good penetration into the uterus. Abdominal obesity or a retroverted uterus may cause difficulty during transabdominal approach. Both transabdominal and transvaginal approaches require the use of an acoustic gel.
A physician typically selects the appropriate estimated delivery date for a pregnant patient. In certain circumstances, an ultrasound technician will be the first person to evaluate a pregnancy using ultrasound. Ultrasound reported estimated date of delivery, as well as other dating methods, should be compared by the treating clinician to choose the best clinical estimate of gestational age using the rules described below.
Patient preparation before ultrasound varies depending on which approach is used. For transabdominal ultrasound, a full bladder is helpful but not required. 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.
Abdominal ultrasound approach may be performed in the supine position. The transvaginal approach should be performed 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 is by using the patient's last menstrual cycle. The patient must be sure of the first day of their last menstrual period to use this method in establishing the due date. Adding seven days and then nine months to the patient’s last menstrual period (or 280 days) will give an estimated delivery date. This technique assumes that the patient has a normal 28-day menstrual cycle and ovulates on day 14 of that cycle.
Fundal height measurement is a physical exam parameter that can be used to estimate gestational age. 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 recording the trend of this measurement to screen for appropriate fetal growth throughout gestation. The usefulness of fundal height measurement in any circumstance has varied widely throughout the literature but can be helpful in resource-poor areas for an estimation of gestational age. The assumption with fundal height measurement is that the measurement in centimeters from uterine fundus to pubic symphysis is equal to the patient's gestational age. Uterine fibroids, amniotic fluid abnormalities, increased maternal body mass index (BMI), and fetal growth abnormalities are some examples of circumstances that can alter the accuracy of fundal height measurement.
First-trimester ultrasound (ultrasound before 13 weeks and 6/7 days) is the most accurate method to establish or confirm gestation age in pregnancy. First-trimester ultrasound can be performed either trans-vaginally or trans-abdominally. Crown-rump length is used for pregnancy dating the first trimester. The average of three crown-rump length measurements is used to improve accuracy. When the crown-rump length exceeds 84 mm (approximately 14 weeks and 0/7 days), the accuracy decreases, and full fetal biometry should be used to approximate the gestational age. First-trimester ultrasound has an accuracy of +/- 5 to 7 days. Last menstrual cycle, if known, should be used to estimate the gestational age before an ultrasound. If the ultrasound is performed at less than 9 0/7 weeks, and the ultrasound dating differs by less than or equal to five days, the last menstrual period should be used for gestational age determination. If the estimated date of delivery in this circumstance differs by more than five days, the ultrasound determined estimated date of delivery should be used. An ultrasound performed between 9 0/7 weeks and 13 6/7 weeks, can differ by seven days. If the ultrasound-determined estimated date of delivery differs by more than seven days, the ultrasound-estimated date of delivery should be used. If the ultrasound-estimated date of delivery differs by less than seven days, the last menstrual period should be used.
The performance of a first-trimester ultrasound is not always possible. Patients occasionally initiate prenatal care in the second trimester, or they may not present to a facility with ultrasound capability. In resource-poor areas, an initial ultrasound should be performed between 18 to 20 weeks. Ultrasound between 18 to 20 weeks will allow both optimal dating criteria and detailed anatomical survey of the fetus. Second-trimester ultrasound estimates estimated date of delivery with fetal measurements of biparietal diameter, head circumference, abdominal circumference and femur length.
The accuracy of second-trimester ultrasound (between 14 0/7 weeks and 27 6/7 weeks) is widely variable. The earlier in the second trimester that an ultrasound is performed, the more accurate gestational age measurement. If a first-trimester ultrasound has been used to confirm or establish an estimated date of delivery, a second-trimester ultrasound should not be used to adjust the estimated date of delivery. If ultrasound is performed between 14 0/7 and 15 6/7 weeks and the date of delivery as estimated by the last menstrual period differs by more than seven days, the ultrasound-estimated date of delivery should be used for pregnancy management. If ultrasound is performed between 16 0/7 and 21 6/7 weeks and the estimated date of delivery by last menstrual period differs by more than ten days, the ultrasound-estimated date of delivery should be used. Pregnancies without confirmation or revision of gestational age by ultrasound before 22 0/7 weeks are considered sub-optimally dated. Beyond 22 0/7 weeks until 27 6/7 weeks, if the last menstrual period-determined estimated date of delivery differs by more than 14 days, the ultrasound estimated date of delivery should be used.
Third-trimester ultrasound (beyond 28 0/7 weeks) is the most inaccurate method for pregnancy dating with an accuracy of +/- 21-30 days. One major concern with third trimester dating ultrasound is underestimating the gestational age of a growth-restricted fetus. Management decisions based on third-trimester ultrasound alone can be difficult for this reason.
Once the estimated date of delivery is established and confirmed with first or second-trimester ultrasound, it should be carefully documented in the medical record for use by other health care providers if needed. Changes to the estimated delivery date can have significant implications for pregnancy management, so before making a change to the patient's estimated date of delivery, the patient should be counseled on possible implications.
Establishing an accurate gestational age and estimated delivery date is the most important step in the management of any pregnancy. Accurate knowledge of gestational age allows laboratory and screening tests to be performed at the appropriate time in the pregnancy. Optimal dating, before 22 0/7 weeks, will enable accurate assessment of fetal growth as the pregnancy progresses. Sub-optimally dated pregnancies, due to the error of ultrasound at advanced gestational age, can be difficult to manage because of the uncertainty of the pregnancy dating. Elective delivery should not be performed in sub-optimally dated pregnancies. In pregnancies with clear medical indications for delivery (pre-eclampsia, gestational diabetes, etc.), delivery planning should be based on the best clinical estimation of gestational age. Amniocentesis for fetal lung maturity should not be used routinely before planning delivery for sub-optimally dated pregnancy. Even with proven fetal lung maturity, late preterm and early term infants have an increased risk of respiratory morbidity. Elective delivery should be performed at 41 completed weeks, due to concerns that the fetus could be further along than estimated by third-trimester ultrasound. Antepartum fetal testing can be performed after 39 weeks in patients with sub-optimal dating due to concerns for post-term pregnancy. In a sub-optimally dated pregnancy, a repeat low transverse cesarean delivery, if desired by the patient, should be performed at 39 weeks based on the best estimate of gestational age.
Dating of pregnancy is done by the primary care provider, midwife, nurse practitioner, obstetrician and the obstetric nurse. Establishing an accurate gestational age and estimated delivery date is the most important step in the management of any pregnancy. Accurate knowledge of gestational age allows laboratory and screening tests to be performed at the appropriate time in the pregnancy. Optimal dating allows one to follow the pregnancy, anticipate any difficulties and predict the day of delivery. The more prepared the pregnancy team is, the better are the outcomes. (level V)
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