I/II/III trimester
I/II/III Trimester
A “normal,” full-term pregnancy is 40 weeks and can range from 37 to 42 weeks. It’s divided into three trimesters. Each trimester lasts between 12 and 14 weeks, or about 3 months.
As you may be experiencing now, each trimester comes with its own specific hormonal and physiological changes.
Being aware of the ways that your growing baby is affecting your body will help you better prepare yourself for these changes as they happen. It’s also helpful to be aware of the specific risk factors (and associated medical tests) for each of the trimesters.
First-Trimester Ultrasonography
First-trimester ultrasound biometry can be used to determine the estimated due date, assess chorionicity in multiple gestations, and assess risk for aneuploidy. At 11–14 weeks of gestation, the combination of nuchal translucency and serum markers (pregnancy-associated plasma protein A, and free or total β-hCG) has been demonstrated to be an effective screening test for Down syndrome.6 The First- and Second-Trimester Evaluation of Risk Research Consortium trial,7 the largest prospective U.S. study of first-trimester risk assessment, found that first-trimester risk assessment provides efficient Down syndrome risk assessment with detection rates of 87%, 85%, and 82% at 11, 12, and 13 weeks, respectively, that is superior to the second-trimester quadruple screen at a fixed false-positive rate of 5% (ie, 5% of unaffected pregnancies will have a positive test result).
Second-Trimester Ultrasonography
The sensitivity of ultrasound diagnosis of fetal anomalies requires additional study beyond the now 15-year-old RADIUS trial. In the Eurofetus study,12 56% of 4,615 malformations were detected, and 55% of the major abnormalities were detected at 24 or fewer weeks of gestation. A screening 18-week ultrasonogram has de facto become the expected standard of care among most obstetricians despite the RADIUS study, but a U.S study has yet to be done to support this clinical conclusion. Outcome data are needed to determine the accuracy and effect of prenatal diagnoses of fetal anomalies and to provide better opportunities for prenatal treatment. Most anomaly scans are done at 18–20 weeks of gestation. Some anomalies are missed or may develop later, such as microcephaly, renal anomalies, dysgenesis of the corpus callosum, hypoplastic left heart, and achondroplasia. Methods to standardize the ultrasound examination to make it less operator-dependent and more efficient, as well as increased accessibility of ultrasonography in remote communities, are areas that require evaluation.
Three-Dimensional Ultrasonography
Currently, a minority of imaging specialists routinely utilize three-dimensional ultrasound technology. Experienced ultrasonographers acquire two-dimensional slices in series and mentally build a three-dimensional image. Goncalves et al17 reviewed 525 articles on three-dimensional/four-dimensional ultrasonography and found that three-dimensional ultrasonography provides additional diagnostic information for the diagnosis of facial anomalies, especially facial clefts, neural tube defects, and skeletal malformations. However, overall studies comparing two-dimensional and three-dimensional ultrasonography for the diagnosis of congenital anomalies have not demonstrated a difference in detection rates. Prospective study of 99 fetuses demonstrated that the sensitivity and specificity of three-dimensional/four-dimensional ultrasonography and two-dimensional ultrasonography for congenital anomalies were not significantly different.