First versus Second Trimester Ultrasound: The Effect on Pregnancy Dating and Perinatal Outcomes
If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound. If the ultrasound date is within. Ultrasound is an essential tool for evaluating your baby Whether or not you receive an ultrasound during your first trimester of pregnancy of the date your last menstrual period (LMP) began. First trimester ultrasound is not recommended to diagnose pregnancy, to date pregnancy when last normal menstrual period and physical examination are.
It allows him to monitor your baby's growth and track milestones, detect abnormalities, home in on your due datedetermine whether you're carrying multiples, see the position of your placenta important for deliveryand a big one for parents-to-be!
Another exam that utilizes ultrasound is Doppler fetal monitoring.
Ultrasound: A Trimester-by-Trimester Guide
This test is typically performed during the last trimester on women who suffer from gestational diabetes. A regular ultrasound uses sound waves to produce images; this one bounces high-frequency sound waves off circulating red blood cells to measure blood flow and blood pressure.
The test will determine if Baby is getting enough blood. An ultrasound is considered safe for both you and your baby when it's used for medical purposes. A trained professional who can interpret the results with accuracy and who is a pro at detecting abnormalities should perform it.
Your technician should be schooled in obstetrical ultrasound, preferably at a center accredited by the American Institute of Ultrasound in Medicine. Unlike X-rays, an ultrasound involves no radiation. It takes 40 weeks for your baby to grow from the size of a tiny seed to a plump watermelon.
Watch it all happen in less than two minutes.
The use of first trimester ultrasound.
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. 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. As soon as data from the last menstrual period LMPthe 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. Introduction An accurately assigned EDD early in prenatal care is among the most important results of evaluation and history taking. This information is vital for timing of appropriate obstetric care; scheduling and interpretation of certain antepartum tests; determining the appropriateness of fetal growth; and designing interventions to prevent preterm births, postterm births, and related morbidities.
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 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.
Ultrasound: A Trimester-by-Trimester Guide
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.
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