Ultrasound: A Trimester-by-Trimester Guide
One of the more frustrating situations occurs when there is a significant difference between a late second- or early third-trimester ultrasound EDD and the. The accurate determination of a patient's "due" date, referred to by doctors and Many patients believe that the later in pregnancy an ultrasound is performed, and who did not get an exam or ultrasound until the third trimester, can have an. With rare exception, if a first-trimester ultrasound examination was performed, in the third trimester and suggests a discrepancy in gestational dating of more.
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. 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.
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.
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.
Ultrasound: A Trimester-by-Trimester Guide
New charts for ultrasound dating of pregnancy and assessment of fetal growth: Ultrasound Obstet Gynecol ; First- and second-trimester ultrasound assessment of gestational age. First- vs second-trimester ultrasound: Br J Obstet Gynaecol ; Gestational age in pregnancies conceived after in vitro fertilization: Ultrasound dating at 12—14 weeks of gestation.
A prospective cross-validation of established dating formulae in in-vitro fertilized pregnancies. Accuracy of gestational age estimation by means of fetal crown—rump length measurement.
First Trimester You missed your period. Your breasts are sore and you're exhausted. You take a home pregnancy test and a plus sign appears. Time to get "official" confirmation at the ob-gyn's office with a blood test and an ultrasound exam. What to expect at your exam Your first ultrasound, also known as a sonogram, will take place when you're around 6 to 8 weeks pregnant. When you schedule your appointment, be sure to ask whether you need a full bladder for the test.
Sound waves travel better through liquid, so a full bladder can enhance the quality of your ultrasound. As your uterus and the fetus grow and you have more amniotic fluida full bladder matters less.
- Women's Health Care Physicians
At this stage, your baby is very small and your uterus and fallopian tubes are closer to your birth canal than to your abdomen, so your ob-gyn will conduct the test transvaginally to get a clearer picture. The test is painless.
Your ob-gyn will place a thin, wand-like transducer probe, which transmits high-frequency sound waves through your uterus, in your vagina.
The sound waves bounce off the fetus and send signals back to a machine that converts these reflections into a black and white image of your baby. It will be hard to see much in this first snapshot, but a clearer photo will come around 13 weeks, which is the ideal time to share your exciting news. Why it's important Your ob-gyn listens for your baby's heartbeat and estimates his age by measuring his length from head to bottom; the baby is tiny in the first trimester and is growing about a millimeter a day.
From this test, your doctor will be able to determine a more accurate due date and track milestones during your pregnancy.
Methods for Estimating the Due Date - ACOG
Your ob-gyn will also rule out a tubal ectopic pregnancy, which is when the fetus grows in the fallopian tube instead of the uterus. This occurs only 1 percent of the time. And your doctor will also be able to tell if you're pregnant with multiples. Other tests All pregnant women are offered a nuchal translucency NT test, performed between 11 and 13 weeks, and this involves another ultrasound.
The NT evaluates your risk of having a baby with Down syndrome, trisomy 18 another chromosomal abnormalityor certain heart defects.
In this two-part exam, a blood test measures levels of certain hormones and proteins in your body, and an ultrasound determines the thickness at the back of baby's neck increased thickness indicates that he may be at risk for birth defects such as Down syndrome and trisomy