Download 1. To determine accurate pregnancy dating. 2. To identify an agreed

University of Illinois Hospital & Health Sciences System
Chicago, IL
NO.: P 1.40
DATE: September, 2008
REVISED: December, 2011
PAGE: 1 of 3
OBSTETRICS GUIDELINE
____________________________________________________________________________
SUBJECT:
PREGNANCY DATING
OBJECTIVE
1. To determine accurate pregnancy dating.
2. To identify an agreed upon schema for determining pregnancy dating so that the
department agrees on management
POSITION STATEMENTS
Pregnancy dating will be by ultrasound if clinical dating is uncertain or if there is a
discrepancy between clinical dating and ultrasound dates. As ultrasound dating early in
pregnancy is very reliable and has a very small margin of error, ultrasound is more
accurate. If it is anticipated that highly accurate dating will be important in the
pregnancy, an early ultrasound should be obtained and used for dating. Alternatively
many patients are having first trimester ultrasound for sequential screening, and this is
an opportunity for more accurate dating.
As more patients are having infertility treatments, they may have accurate clinical dating
by a known ovulation or embryo transfer date. This is the most accurate dating
available.
Importance of reliable pregnancy dating:
A.
Avoid iatrogenic newborn prematurity
B.
Determine when to initiate post term fetal surveillance testing
C.
Facilitate decisions regarding delivery timing and treatment of preterm
labor or preterm premature ruptured membranes.
D.
Appropriate timing of gestational age-related testing
PROCEDURE
KEY ACCOUNTABILITIES:
1. The physician/midwife provider will initiate pregnancy dating procedures at the first
pregnancy encounter.
2. The physician/midwife provider will evaluate pregnancy dating reliability considering
the following evidence based guidelines, documenting adjustments in the pregnancy
record:
A. Dating will be by ultrasound if clinical dating uncertain
University of Illinois Hospital & Health Sciences System
Chicago, IL
NO.: P 1.40
DATE: September, 2008
REVISED: December, 2011
PAGE: 2 of 3
OBSTETRICS GUIDELINE
____________________________________________________________________________
B. Dating by clinical if infertility patient and date of conception is clear
C. Dating will be re-assigned to ultrasound dates from clinical dates (by last
menstrual period [LMP]) depending on gestational age and discrepancy between
LMP and ultrasound using the general evidence based guidelines in TABLE 1 as
follows:
[NOTE: Dating changes made after Quad screen performed may require
recalculation of prior results.]
TABLE 1:
Procedure for changing the EDC when the ultrasound and LMP data do not match
Dating by
Ultrasound
7 0/7 < 9 6/7 wks
Difference from
LMP (days)
≥ 3 days
10 0/7 - 13 6/7 wks
≥ 5 days
If 14 0/7 weeks by LMP and 13 2/7 weeks by
ultrasound, re-dating recommended.
14 0/7 – 21 6/7 wks
≥ 7 days
If 19 0/7 weeks by LMP and 18 0/7 weeks by
ultrasound, re-dating recommended.
22 0/7 – 25 6/7 wks
≥ 10 days
If 22 2/7 weeks by LMP and 24 2/7 weeks by
ultrasound, re-dating recommended.
26 0/7 - 29 6/7 wks
≥ 14 days
If 30 0/7 weeks by LMP and 28 0/7 weeks by
ultrasound, re-dating recommended.
≥ 30 wks
Example
If 7 0/7 weeks by LMP and 7 4/7 weeks by
ultrasound, re-dating recommended.
ALWAYS re-date with caution. In general, this
must be individualized. Need to consider IUGR
as an explanation for the discrepancy. Consider
re-dating if there is a large difference (i.e. ≥ 3
weeks).
REFERENCES:
Variability is uniform at 8% for crown rump length [CRL] measurements between 2mm
and 12cm: (Hadlock 1992)
University of Illinois Hospital & Health Sciences System
Chicago, IL
NO.: P 1.40
DATE: September, 2008
REVISED: December, 2011
PAGE: 3 of 3
OBSTETRICS GUIDELINE
____________________________________________________________________________
Accuracy of CRL in predicting menstrual age was 3-5 days: Sonar measurement of fetal
crown-rump length as means of assessing maturity in first trimester of pregnancy. BMJ
1973;4:281. Robinson HP, Fleming JE.
A critical evaluation of sonar "crown rump length" measurements. Br J Obstet Gynaecol
1975;82:702. Drumm JE, Clinch J, MacKenzie G.
The ultrasonic measurement of fetal crown-rump length as a method of assessing
gestational age. Br Obstet Gyneaecol 1976;83:417
Accuracy of ultrasound in predicting gestational.age is +/- 7 days for 12-18 weeks:
Reece EA, Gebrielli S, Degennaro N, et al. Dating through pregnancy: a measure of
growing up. Obstet Gynecol Surv. 1989;44:544.
Callen, PW. Ultrasonography in Obstetrics and Gynecology; 5th edition, 2008.
Hadlock FP, Harrist RB, Martinez-Payer J. How accurate is second trimester fetal
dating? J Ultras Med 10:557,1991.
Degani S. Fetal biometry: clinical, pathological and technical considerations. Obstet
Gynecol Surv 56:159,2001.
Chervenak FA, Skupski DW, Romero R, et al. How accurate is fetal biometry in the
assessment of fetal age. Am J Obstet Gynecol 178:678,1998.
Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: Ultrasound
biometry versus certain menstrual dates. Am J Obstet Gynecol 174:278,1996
Fleisher A, Toy E, Lee W, Manning F, Romero R. Sonography in Obstetrics and Gynecology:
Principles and Practice, 7th Edition, McGraw-Hill, 2011.
______________________________
Judith Hibbard, M.D.
Medical Director, Obstetrics
Professor, Obstetrics & Gynecology
Director, Maternal Fetal Medicine
DATE: ______________________________