Methods for Estimating the Due Date - ACOG
First trimester ultrasound is the most accurate time frame for pregnancy dating and can increase the accuracy of the EDD even if LMP is known. The last menstrual period (LMP) was considered certain in 13, and uncertain in cases. The duration of pregnancy from the scan to the day of. Calculate delivery due date, gestational age and other important dates during pregnancy using LMP, ultrasound dating, or date of conception (IVF).
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.
Predicting delivery date by ultrasound and last menstrual period in early gestation.
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.
How Doctors Date Pregnancies, Explained
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. Ultrasound dating is based on a series of measurements of the gestational sac in very early pregnancies and of the embryo or fetus itself as pregnancy progresses.
How Doctors Date Pregnancies, Explained - negeriku.info
However, unless there is a great difference between what is seen on an ultrasound and what was reported as LMP, doctors still use LMP to establish a due date. For example, if in the first trimester the ultrasound estimate falls within one week of the LMP estimate, we still use LMP to determine due date and length of pregnancy. If the ultrasound estimate differs by more than one week from the LMP, we would use the ultrasound estimate for the due date.
This method continues in the subsequent trimesters. In the second trimester, the determination also defaults to the LMP unless the ultrasound estimate is more than two weeks different from the LMP.
We follow the same process in the third trimester, with a three-week difference. Physical Exam Pregnancies can also be dated with a physical exam.
The uterus then typically rises above the navel at about a centimeter a week after that. But a physical exam can be misleading due to factors such as twins, uterine tumors, or obesity and is not used as the main way to date a pregnancy if LMP or ultrasound is available.
Ultrasound in twin pregnancies. J Obstet Gynaecol Can. Sperling L, Tabor A. Acta Obstet Gynecol Scand. Method for estimating due date. Ameri-can College of Obstetricians and Gynecologists. The timing of the "fertile window" in the menstrual cycle: Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby. Determination of Gestational Age by Ultrasound No. J Obstet Gynaecol Can ;36 2: Martin JA, et al.
Natl Vital Stat Rep. Page JM, The risk of stillbirth and infant death by each additional week of expectant management in twin pregnancies. Epub Mar Risk of late-preterm stillbirth and neonatal morbidity for monochorionic and dichorionic twins.
Epub Mar 5. Definition of term pregnancy.