pregnancy dating, assessment gesational age

Importance of reliable pregnancy dating:

 Appropriate timing of gestational age-related testing
 Avoid iatrogenic newborn prematurity
 Decisions regarding delivery timing and treatment of PTL
or PPROM
 Determine when to initiate post term fetal surveillance

2/15/2013 10:00 AM/ dr.veerendrakumar cm 2

 Pregnancy dating methods
 Pitfalls
 Dating discrepancy
 Judicious use of technology for better
outcome
 Learn to critically analyze dating !!!


 Social and medical significance

 Time frame around which many activities
are planned


 when dates are inaccurate, antenatal test results
will be incorrect and misleading.

Cowchock FS, Clin Obstet Gynecol 19: 871, 1976


Obstetric management is dependent on gestational age

Goldenberg RL, Am J Obstet Gynecol 123: 617, 1975


Elephant- 640 days
Dogs- 63 days
Cat- 65 days
Monkey- 164 days
Mouse- 21 days
Sheep- 150 days
Cow- 280 days
Human- 266 days

DO WE MATURE EARLY?

 Does gestation vary by ethnic group? A London-based
study of over 122 000 pregnancies with spontaneous
onset of labour

Roshni R Patel, Philip Steer,
I J E VOL 33 No.1 107-113 ,2004


Metabolic nirvana ….
 The median gestational age at delivery was 39 weeks in
Blacks and Asians and 40 weeks in white Europeans.

 Meconium stained amniotic fluid, which is a sign of fetal
maturity, was statistically significantly more frequent in
Black and Asian infants compared with white European
infants.


The age…
1.Menstrual age

2.Gestational age

3.Embryological age

1 22 3


Basic methods
 Menstrual history

 Clinical examination

 Ultrasound examination


Menstrual history
 Indirect measure of conception of roughly 280
days

 Reliability depends and influenced by
- recall of memory
- presence of light bleeding
- regularity of cycles
- OCP, breast feeding etc
Nichols C. Dating pregnancy. J Nurse Midwifery.
1987;32:195-204. cm
2/15/2013 10:00 AM/ dr.veerendrakumar 12

Menstrual age

 Overestimates the gestational age

Wilcox M et al BMJ 1993;307(6904):588-91

 10-45% do not provide correct information

 18% with certain dates have significant difference
with USG age

Geirsson et al, Br J Obstet Gynaecol. Jan 1991;98(1):108-9.

Naegeles’ rule does not correlate with
actual conception date….

 Baird et al
498 normal cycles
luteal phase 7-19 days
10% ovulate on D14

 Wicox et al
only 30% of normal cycle women are fertile
between 10-17 days

Reliable information on 1st day of LMP not
equivalent with reliable information about
the start of pregnancy

Naegeles revisited..
 19th century Frederiech Naegele original article
in German textbook

 Naegele himself credits Harmann Boerhaave
with Inventing the formula for EDD.

 Its unclear whether 1st day of LMP or

last day of LMP to be considered for EDD.

Other methods
 Prem’s rule –based on BBT

7days subtracted from 1st day of thermal shift and 9
months added

 M G Hiremath (KIMS Hubli)
pt is ignorant about English calendar
doctor is ignorant about Lunar calendar!
 Paper presented- International Conference on labour
and delivery at Mumbai 1994.

Time to discard Naegele’s?
 Error prone EDD not acceptable in modern
times
Katz VL et al Why we should eliminate the due date: A
truth in jest. Obstet Gynecol. 2001;98:1127-1129

Gardosi J. Dating of pregnancy: Time to forget the last
menstrual period. Ultrasound Obstet Gynecol. 1997;9:367-
368

Savitz DA. Comparison of pregnancy dating by last
menstrual period, ultrasound scanning and their
combination. Am J Obstet Gynecol. 2002;187:1660-1666

That too when we know the embryo
like this


2/15/2013 10:00 AM29/12/2009 dr.veerendrakumar cm 21

Ultrasound dating
 RADIUS trial 1993 - 15,000 low risk women

no clinical benefit to justify routine ultrasound in the
absence of medical indications

ACOG 2008 states that obstetricians are under no
obligation to perform routine ultrasounds in low-risk
women; however, it is reasonable to honor a patient's
request for one.!!

Ultrasonography in pregnancy. Obstet Gynecol.
2008;112:951-962.


Obstetric ultrasound turned into
Ultrasonic obstetrics

Sono obstetrician

RADIOLOGIST
CAN SURVIVE
WITHOUT SCAN

BUT
NOT THE

OBSTETRICIAN

2/15/2013 10:00 AM dr.veerendrakumar cm 24

All said …

 Ultrasound examination of the fetus provides the most
precise assessment of gestational age, especially in the
first trimester.

Callen USG in Obst and Gynecol 2008
ACOG PRACTICE BULLETIN 2008


9 RCT, > 34,000 women

 Routine USG

More accurate estimation of EDD
earlier detection of twin pregnancies

Neilson JP , Cochrane Database –Syst Review 2000

improved clinical management of parturient women


 inaccuracies of history and physical
examination may limit their usefulness in
assessment of gestational age.


• In vitro fertilization the most accurate means of
predicting gestational age ( 1 day).

 in most pregnancies, the date of ovulation or
conception cannot be accurately predicted and
gestational age must be established by other
methods.


Methods of determining the
gestational age
 Parameter variability 2SD
IVF 1 day
Ovulation 3 days
IUI 3 days
Single I/C 4 days

1st trimester examn 2 weeks
2nd trimester examn 4 weeks
3rd trimester examn 6 weeks
Callen PW. Ultrasonography in obstetrics and gynecology. 5th ed.
Saunders Elsevier; 2008;p. 3-25.

Methods of determining the
gestational age
 Parameter Variability 2SD
1st trimester USG –CRL

± 8% of the estimate

2nd trimester USG –HC,FL

3rd trimester USG – HC,FL


Callen PW. Ultrasonography in obstetrics and gynecology. 5th ed. Saunders
Elsevier; 2008;p. 3-25.

USG technology evolved..
 The margin of error is consistently ±8%

by esteemed researchers

Hadlock FP, Radiology. 1992;182:501-505

Doubilet ,J Ultrasound Med.1993;12:647-653

Taipale P, Obstet Gynecol. 2001;97:189-94


In the first trimester
 MSD and CRL- primary means of evaluating
gestational age.

Am J Obstet Gynecol 103: 789, 1969
Acta Obstet Gynecol Scand 50 (suppl): 29, 1971
Br J Obstet Gynaecol 82: 702, 1975


Usg in First trimester
 Gestational Sac (GS) - MSD

used till embryo appears

 add 30 to GS size in mm to get age in days


The sac is measured inside the hyperechoic
rim, including only the echo-free space.


measurement of the fetal length from the
tip of the cephalic pole to the tip of the
caudal pole.

 The embryo is measured along its longest axis to
obtain the CRL measurement accurately date
pregnancy between 7 and 13 weeks' gestation.

 The fetus should be at rest and assuming its
natural curvature.


CRL


Technical factors can lead to
errors in CRL measurements.

- incorporation of the yolk sac or lower limbs in the
CRL measurement,
- excessive curling or extension of the fetus,
- tangential section of the trunk.

Robinson HP, A critical evaluation of sonar “crown-
rump length” measurements. Br J Obstet Gynaecol
82: 702, 1975

Overestimation…

17 mm- 8 weeks 22 mm -8 wks 5 days


Excessive flexion -underestimation

54 mm- 11wks 4 days 62mm- 12wks 4days

2/15/2013 10:00 AM29/12/2009 dr.veerendrakumar cm 39

 The BPD is imaged in the transaxial plane of the
fetal head at the level of thalami in the
midline, equidistant from the temporoparietal
bones and usually the cavum septum pellucidum
anteriorly.

 Linear correlation with BPD, HC

racial difference with FL, not with HC

GA (DAYS) =dr.veerendrakumar cm (BPD)in mm
39.1 + 2.1
2/15/2013 10:00 AM/ 40

 Between 12 and 26 weeks' the BPD is accurate ±10
to 11 days.

 After 26 weeks' the accuracy of BPD measurement
progressively decreases and is ±3 weeks near term.


 BPD measurement is most accurate in assessing gestational
age when the head shape is appropriately ovoid.
 If the head is rounded (brachycephalic) or elongated
(dolicocephalic), BPD measurements would overestimate or
underestimate gestational age, respectively.

 in fetuses with abnormal head shape, either brachycephaly
or dolicocephaly, HC may be a more accurate predictor of
fetal age than BPD.
Hadlock FP, : Fetal head circumference: Relation to
menstrual age. AJR 138: 649, 1982

Technical factors
influence the accuracy.

 Inter observer error,

different techniques of measurements, and

single versus multiple measurements

The limitations of ultrasonic fetal cephalometry. J
Obstet Gynaecol Br Commonw 80: 769, 1981



AC is obtained..
 in the trans axial view of the fetal abdomen,

at the level of the fetal liver,
the umbilical portion of the left portal vein
& fetal stomach which is at the same level


AC is less accurate than head
measurements (BPD or HC).

 AC is the growth parameter most commonly
affected in pregnancies complicated by abnormal
fetal growth patterns.

 Hadlock FP, Deter RL, Harrist RB, Park SK: Fetal
abdominal circumference as a predictor of menstrual age.
AJR 139: 367, 1982


GASA and Multiple parameters

 To improve the accuracy of gest age- assessment
growth-adjusted sonographic age ( GASA) and
averaging multiple parameters are used.

Sabbagha RE, GASA: A simplified method. Obstet Gynecol
51: 383, 1978
Hadlock FP, Am J Obstet Gynecol 156: 955, 1987


 Gestational age can be more accurately predicted by
obtaining paired BPD measurements
 (the first from 20 to 26 weeks' gestation and the
second from 31 to 33 weeks' gestation)

 and assigning gestational age by a method known as
growth-adjusted sonographic age (GASA).


 Use of GASA based on specific cephalic growth
pattern increases the accuracy of gestation by BPD
measurement to within ±3 to 5 days.

Obstet Gynecol 51: 383, 1978


Femur length
 The largest of the long bones, least moveable, and
easiest to image.

 The ultrasound beam should be perpendicular to
the shaft.

 The measured ends should be blunt in appearance
and the distal femoral epiphysis should not be
included.


 FL accurately predict gestational age between 14
weeks' and term.

 Jeanty P, J Ultrasound Med 3: 75, 1984


Biologic and technical factors may lead to
inaccuracies of FL measurements.

Overestimation of FL, particularly when the femur
is in the far field or lateral margins of image.

 Tangential section of the femur, failing to
visualize the entire length of the shaft, leads to
underestimation of FL


 The distal femoral epiphysis becomes echogenic in
the third trimester, Inclusion of the distal epiphysis
will falsely overestimate FL.

Goldstein RB, Filly RA, Simpson G: Pitfalls in femur
length measurements. J Ultrasound Med 6: 203, 1987


The FL/BPD ratio (normal 79 ± 6%)

 useful as an internal verification of the
measurements obtained.
Abnormal ratio is an indicator of pathologic entities,

 Microcephaly
( abnormally high)
 hydrocephalus or short-limb dysplasia
(abnormally low).


Multiple Fetal Growth
Parameters
 If the gestational age estimates derived from all of the
parameters are similar, assignment of gestational
age from the average of all the parameters will
improve accuracy.
 If gestational age estimates of the various parameters
are quite different, averaging multiple parameters
will decrease the accuracy of the best predictor(s).


Averaging of fetal growth parameters
should be avoided
 fetal macrosomia,
 intrauterine growth retardation (both symmetric
and asymmetric),
 congenital anomalies (skeletal dysplasia,
hydrocephalus and others)


 Assessment of gestational age is fundamental to
obstetric care and should be a carefully thought-out
process.
 Assessment should depend on history and physical
examination, as well as ultrasound evaluation.

 Ultrasound is a reliable method for establishing the
length of pregnancy and in this way can improve
obstetric care.

SCAN IS A WONDERFUL
INSTRUMENT IN THE RIGHT HANDS


 an ultrasound performed before 24
weeks' gestation

establishes a more accurate EDD than
relying solely on the LMP.

Hadlock et al, Mongelli et al


Despite RADIUS….
 In study of 17,000 women

With reliable LMP dating post term delivery rate
reduced from

10.3% to 2.7%
Taipale P, Hiilesmaa V. Predicting delivery date by
ultrasound and last menstrual period in early gestation.
Obstet Gynecol. 2001;97:189-194.

Dating discrepancy
 Problem arises in (busy) clinical practice
when significant difference exist between
due date calculated by LMP and USG

especially when earlier scan is not done and
patient presents late in pregnancy.


Ultrasound biometry
‘approximation of assumption’
 In early pregnancy

all fetuses of same age are of same size !
Variation increases with increase in gestational age

 In late pregnancy

different fetuses of same age are of different
sizes

Reassigning the due date
 ACOG recommends reassigning the date

when difference exist between LMP & USG date.

>7days difference up to 12 weeks

> 10 days difference 12-20 weeks

What about after 20 weeks?

Redating..


Man or machine?
Or..
Middle path


The classical understanding…
 1st trimester ± 1 week

2nd trimester ± 2 weeks

3rd trimester ± 3 weeks


Rule of 8
 Hadlock regression analysis

USG gestational age estimation is consistently accurate
with 8% margin of error irrespective of the GA.

This SD is to be converted into days’ gestation

Hadlock F. Sonographic estimation of fetal age and
weight. Radiol Clin North Am. 1990;28:39-50.

±8 % variance (Hadlock) of the
estimated gestational age


not in weeks but convert into days!

8weeks = 56 days » 8% = 5 days

24 weeks =146 days » 8%=12 days

36weeks = 252 days » 8% = 20 days


The problem..
 LMP 10/01/03 EDD 17/10/03

USG done on 1/9/03
gest age of 29 weeks 6day
EDD 10/11/03

 Difference is 24 days !
which one to consider?

 If difference is more than MOE -use USG EDD

 If MOE is more than difference use LMP EDD


 Convert gestational age into days

29wks 6day = 209 days

 8% of Margin of error (MOE)

= 209 x 0.08 = 16.7 = 17 days

 final EDD is 10/11/03


The baby was born on 2/11/2003


 Use always earliest USG report

 Subsequent USGs can be used to rule out developmental abnormalities.

 Uniformity among obstetric providers without divergent opinion in
arriving at Final EDD

 Busy tertiary care hospital consistency is crucial

 Allows residents to learn critically learn & examine the dating criteria

 Evidence based management with confidence


pregnancy dating, assessment gesational age

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