- Determining gestational age and chorionicity - Multiple Pregnancy - NCBI Bookshelf
- Women's Health Care Physicians
- Multiple Pregnancy: The Management of Twin and Triplet Pregnancies in the Antenatal Period.
- Methods for Estimating the Due Date
Appropriately performed obstetric ultrasonography has been shown to accurately determine fetal gestational age 1. A consistent and exacting approach to accurate dating is also a research and public health imperative because of the influence of dating on investigational protocols and vital statistics. This Committee Opinion outlines a standardized approach to estimate gestational age and the anticipated due date.
However, there is great usefulness in having a single, uniform standard within and between institutions that have access to high-quality ultrasonography as most, if not all, U. Accordingly, in creating recommendations and the associated summary table, single-point cutoffs were chosen based on expert review. 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 instance, the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo and the date of transfer. For example, for a day-5 embryo, the EDD would be days from the embryo replacement date.
Likewise, the EDD for a day-3 embryo would be days from the embryo replacement date.
Determining gestational age and chorionicity - Multiple Pregnancy - NCBI Bookshelf
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.
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. 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.
For the purposes of research and surveillance, the best obstetric estimate, rather than estimates based on the LMP alone, should be used as the measure for gestational age. Furthermore, the latter policy would also have the in vitro fertilization pregnancies. Fetal crown—rump nicians place less emphasis on CRL discrepancy, which is length: Comparison of methods for determining crown—rump significant twin growth discordance.
J Clin Ultrasound ;9: The ultrasonic measurement of None to disclose. Ultrasound dating at 12—14 weeks of gestation A pro- TD collected data and wrote the manuscript. SM-D wrote spective cross-validation of established dating formulae in in-vitro the manuscript. BT conceived the idea and wrote the man- fertilized pregnancies. Ultrasound Obstet Gynecol ; AP wrote the manuscript. Fetal size and dating: Intertwin Details of ethics approval disparity in fetal size in monochorionic and dichorionic pregnancies.
The retrospective nature of this observational study did not Obstet Gynecol ; Growth discrepancy in twins in the first trimester of pregnancy. There was no funding for this study. Relationship of intertwin crown-rump length discrepancy to chorio- nicity, fetal demise and birth-weight discordance. Ultrasound Obstet Acknowledgements Gynecol ; Fetal volume and crown-rump length from 7 to 10 weeks of sonographic measurement of crown-rump length. Ultrasound Obstet gestational age in singletons and twins.
Estimation of gestational age by trans- N, et al. Evaluation of measurement of fetal crown-rump length vaginal sonographic measurement of greatest embryonic length in from ultrasonically timed ovulation and fertilization in vitro. Asia dated human embryos.
Ultrasound Obstet Gynecol ;4: Oceania J Obstet Gynaecol ; First- and second-trimester ultrasound assessment of gesta- tional age. Remember me on this computer. Enter the email address you signed up with and we'll email you a reset link. Click here to sign up. Help Center Find new research papers in: The available evidence was limited in quantity and quality.
No randomised controlled trials RCTs were identified and most of the included studies were retrospective in design, using a variety of different methodologies for example, categorical versus continuous representation of gestational age, smaller and larger twins analysed independently or combined, size of fetus used to date pregnancy, head circumference versus crown—rump length.
The quality of evidence for differences in fetal size in twin and triplet pregnancies versus singleton pregnancies was mainly very low. The quality of evidence for differences in dating of twin and triplet pregnancies versus singleton pregnancies was also mainly very low, as was the quality of evidence for prediction of growth discordance and accuracy of dating. The majority of the studies did not report chorionicity or ethnicity. Only one study considered triplets, with the other studies concentrating on twins.
This review question addressed whether there are differences in dating or the size of singleton versus twin or triplet pregnancies that should be taken into account when calculating gestational age in clinical practice. In view of the limitations of the evidence, the GDG based its recommendation on consensus within the group and highlighted the need for further research in this area. The best interval for performing all three tests together is, therefore, when crown—rump length is between 45 mm and 84 mm at approximately 11 weeks 0 days to 13 weeks 6 days.
In practice, it may not be possible to schedule all three tests at the same appointment, and in such circumstances more than one appointment in a short period may be needed. However, if the woman is known in advance to have a twin or triplet pregnancy for example, if such a pregnancy results from IVF treatment it may be possible to plan to schedule all three tests in a single appointment. Evidence suggests that the mean twin measurement best reflects gestational age, both in the first and second trimester , whether using crown—rump length in the first trimester or head circumference in the second trimester.
The GDG recommends using the larger twin measurement to determine gestational age in the first half of pregnancy because using the mean twin measurement would lead to an underestimate of gestational age if the smaller twin were pathologically undergrown. Similarly, the largest triplet measurement should be used to date triplet pregnancies. This guideline specifies the care that women with twin and triplet pregnancies should receive that is additional or different from routine antenatal care for women with singleton pregnancies.
Note that for many women the twin or triplet pregnancy will be detected only after their routine booking appointment. View in own window.
Women's Health Care Physicians
However, the aim in this recommendation is to keep to a minimum the number of scan appointments that women need to attend within a short time, especially if it is already known that a woman has a twin or triplet pregnancy. Pregnancy risks, clinical management and subsequent outcomes are very different for monochorionic and dichorionic twin pregnancies and monochorionic, dichorionic and trichorionic triplet pregnancies. Currently, there appears to be considerable variation and uncertainty in the practice of assigning chorionicity for twin and triplet pregnancies, leading to the GDG prioritising this question for review.
Diagnostic accuracy of various methods for determining chorionicity in twin and triplet pregnancies at different gestational ages was sought. What is the optimal method to determine chorionicity in multiple pregnancies?
No existing NICE guidance was identified as being relevant to this review question. Fourteen studies investigating diagnostic accuracy of the following characteristics as determined by an ultrasound scan for determining chorionicity were identified for inclusion: Only two studies included triplets, and one of these included only one triplet pregnancy, meaning that sensitivity, specificity, positive predictive values PPVs and negative predictive values NPVs and likelihood ratio statistics could not be calculated using the triplet data in the study.
Six prospective cohort studies reported findings for using membrane thickness to determine chorionicity in twin pregnancies. Four prospective cohort studies reported on using the number of placental masses and a lambda or T-sign for determining chorionicity in twin pregnancies. One prospective cohort study reported on using the number of membrane layers to determine chorionicity in twin pregnancies.
One prospective cohort study conducted in the USA reported on using the number of placental sites to determine chorionicity in twin pregnancies. Seven studies reported findings for a mixture of methods for determining chorionicity in twin and triplet pregnancies.
Multiple Pregnancy: The Management of Twin and Triplet Pregnancies in the Antenatal Period.
Evidence profiles for this question are presented in Tables 4. GRADE summary of findings for scans performed at 11—14 weeks of gestation. GRADE summary of findings for scans performed at more than 14 weeks of gestation. GRADE summary of findings for scans performed before 11 weeks of gestation or over a wide range of gestational ages with no mean age reported. Results for twin pregnancies are expressed in terms of detection of monochorionicity. For example, diagnostic accuracy values for the lambda sign are reported as absence of the sign which suggests monochorionicity rather than presence of the sign which suggests dichorionicity.
Results for triplet pregnancies are expressed in terms of detection of a monochorionic or dichorionic triplet pregnancy, rather than a trichorionic pregnancy. Evidence was identified for a variety of methods used to determine chorionicity from ultrasound scans in twin and triplet pregnancies. The sensitivity and specificity of the methods used to determine chorionicity from ultrasound scans is generally high.
The sensitivity for this test was also high. For a mean or median gestational age of more than 14 weeks at the time of scan, results were reported for the use of membrane thickness very low quality evidence , the number of placental sites moderate quality evidence and two different composite methods very low and moderate quality evidence. The highest sensitivity was reported when membrane thickness was included in the composite method.
Methods for Estimating the Due Date
Some studies reported findings for a gestational age of less than 11 weeks or over a wide range of gestational ages with no mean age reported. The composite methods showed the strongest likelihood ratios and high sensitivity. The GDG is aware that the evidence presented may be biased due to analysis after the study concluded for patterns that were not specified before the study, particularly in studies that examined individual methods such as membrane thickness.
In these studies, it is not clear how a clinician determining chorionicity on one measure alone such as subjectively thin or thick membrane would not be influenced by other aspects of the ultrasound scan such as the number of gestational sacs. No published health economic analyses were identified and this question was not prioritised for health economic analysis as part of the development of the guideline. The various measures based on ultrasound scans which were evaluated in terms of diagnostic accuracy could all be obtained from a single scan, and so the costs associated with undertaking individual and composite measures are likely to be similar.
Sensitivity is the percentage of pregnancies found to be monochorionic at placental examination that were predicted to be monochorionic at scan true positive. Specificity is the percentage of pregnancies found to be dichorionic at placental examination that were predicted to be dichorionic at scan true negative.
PPV is the percentage of pregnancies predicted to be monochorionic by the scan that were confirmed at placental examination to be monochorionic. NPV is the percentage of pregnancies predicted to be dichorionic by the scan that were confirmed at placental examination to be dichorionic. The GDG prioritised likelihood ratios and sensitivity when considering the evidence for different methods of predicting chorionicity. Determination of chorionicity is required to correctly stratify perinatal risk according to the type of twin or triplet pregnancy.
Since pregnancy risks, clinical management and subsequent outcomes are very different for monochorionic and dichorionic twin pregnancies and monochorionic, dichorionic and trichorionic triplet pregnancies , accurately determining chorionicity is very important. Monochorionic twin pregnancies have a higher risk of developing complications, including feto-fetal transfusion syndrome FFTS , fetal growth problems, structural abnormalities and overall perinatal loss compared with dichorionic twin pregnancies. The assessment of chorionicity is easier in the first trimester than in later pregnancy and so it is important to assess and document chorionicity clearly at this gestational age.
There is benefit in identifying true positives as women with monochorionic pregnancies will require additional fetal surveillance. Women can make decisions fully informed of risks and appropriate management of monochorionicity can be implemented. Identification of true negatives women with dichorionic pregnancies will result in a saving of time and money by avoiding unnecessary additional interventions.
False positives will result in additional and unnecessary monitoring, anxiety and cost in women with dichorionic pregnancies. False negatives have the least desirable outcome, as monochorionic pregnancies will be monitored less, increasing the likelihood of missing serious complications.
Furthermore women with false negative test results will not be informed about these potential risks and the consequences. The trade-off between clinical benefits and harms is unaffected by the choice of methods for determining chorionicity since any measurements would be taken during a single ultrasound scan appointment.
There is no cost difference between the methods themselves except that composite methods might take more time for measurements to be conducted as they can be done at the same ultrasound scan. A method that is more accurate will be more cost effective than less accurate methods if it means fewer women with dichorionic pregnancies receive unnecessary extra monitoring.
go The GDG emphasised that these scans will tie in to the existing NICE guidance for dating pregnancy and screening, and so the extra costs will be minimal. Only one study reported on diagnosing chorionicity in triplet pregnancies and this study evaluated only one method.
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The GDG assumed that the diagnostic accuracy of methods for determining chorionicity were similar for twin and triplet pregnancies. The GDG is aware that current practice for determining chorionicity involves a composite of methods and there are differences across England and Wales in timing of ultrasound scans.