HOW TO CALCULATE GESTATIONAL AGE

ESTIMATION
OF
GESTATIONAL
AGE
Osama M. Warda MD
Prof. OBS/GYN
Mansoura University

WHY GESTATIONAL AGE ESTIMATION IMPORTANT?
1. Prevention of fetal prematurity (delivery before 37 weeks) as
will as postmaturity (delivery after 42 weeks).
2. Optimizing antepartum fetal growth surveillance; i.e. correlate
the interauterine fetal growth to the gestational age. This is
essential for diagnosis of certain fetal growth problems, e.g.
a) Small for gestational age
b) Intrauterine fetal growth restriction
c) Fetal macrosomia
3. Optimization of Timing of certain fetal tests: (table-next slide)
4. Optimization of the social circumstances of the pregnant
woman & her family regarding travelling, works and other social
activities

The GA can be known by one of the following methods:
A- History Taking
1- Known LNMP-
2- Known single coitus date
3- Known date of Embryo transfer in IVF
4-Known date of Queckening
5- Known early ultrasound
B. Clinical Examination:
1- Bimanual exam in early pregnancy
2-Fundal level
3-Symphysis Fundal Distance
4-Abdominal girth, and fetal tone.
C- Imaging: Ultrasound is the best, and safest. X-ray is no longer used.

HISTORY TAKING
1-Known LNMP:
- it is calculated in weeks using different
methods:
Naegele’s formula: used when the 1st day of the
last normal menstrual period (LNMP) is certainly
known. The expected date of delivery (assuming
that human pregnancy is 40 weeks=280 days) is
calculated by adding 7 days to the days & 9
months to the month.
OR

Example : if the lady informed you that the 1st day of the LNMP
was the 10th of September, 2020,
- EDD will be the [(10/9/2020)+(7 days+ 9 months)
=17th of June 2021. OR by another method
[(10/9/2020)+ (7 days) ]- [(3 months)+1year]=17/6/2021
HISTORY TAKING
The gestational age at the day of case taking can be calculated
by:
[EDD- Date of Today] =X , EDD-X = GA
OR
Date of Today-LNMP= GA

HISTORY TAKING
LNMP= 10/9/2020.
EDD= 17/6/2021. Today is 17/5/2021
The GA is calculated as follows:
(A). 17/5/2021 MINUS 10/9/2020= 8 months+ 7
days = 36 weeks
(B). 17/6/2021 minus 17/5/2021 = 30 days = 4
weeks +.
40 -4= 36 weeks
13 weeks= 3 months
Full term=40 weeks= 9 months + 1 week

HISTORY TAKING
2. Known Single coitus:
- EDD= date of single coitus + 265 day
- The duration of pregnancy can be calculated from the EDD
as before .
- Example: the husband was abroad, he came to cairo as
transit on 20/1/2021. Sexual relation occurred.The wife
reported that she had missed 3 or 4 periods and she felt
pregnant, confirmed by positive home pregnancy test.
- EDD= 20/1/2021+ 265days=14/10/2021
- GA is calculated accordingly as before.

HISTORY TAKING
3- Known date of Embryo transfer in IVF:
GA= (date of ET -7 days )+ 9 months
EXAMPLE :
Date of ET was at 20/4/2021
EDD= (20/4/2021-7 days)+ 9 months = 13/1/2022
The GA can be calculated from the EDD as before.

HISTORY TAKING
4. Date of Queckening :
-Knowing the exact dat of quecknenig; we know the
LMP (APPROXIMATELY) by sbtracting 18 weeks in
multipara or 20 weeks in primigravida from the date of
today.
- Accordingly, the EDD and GA can be calculated.
5- Knowing an early ultrasound:
If the patient has an ultrasoud image early in her current
pregnancy, the EDD, and GA can be easily known by
correlating the GA recorded in the ultrasound to the date
of that ultrasound exam.

CLINICAL EXAMINATION
1- Bimanual examination Early Pregnancy: [in absence of ultrasound ]
- used for both diagnosis of early pregnancy and determination of
gestational age (if the uterus is not felt abdominally).
- When the lower part of the uterus was felt empty & soft while the upper
part was enlarged due to presence of conceptus (Hegar’s sign).
- The uterus with a size as an orange was reported as 8 weeks pregnancy,
while that of a size as a fetal head was reported as 12 weeks pregnancy.
These approximate figures are no longer used in obstetric practice.
However, some gynecologists still use them for reporting uterine
enlargement due to gynecologic cause.

2- Fundal Level
Palpate with ulnar border of
the left hand
QUESTION
Where is your right hand?

3- Symphysis Fundal hieght:

Q1. What are the criteria of LNMP?
Q2. When the LNMP is UNRELIABLE?
Q3. What are causes of uterus/fetus more
than period of amenorrhea?
Q4. What are causes of uterus/fetus
smaller than period of amenorrhea?

4-Abdominal girth, and fetal tone:
In cases where there is no no known dates and
evaluation of GA is only clinicaly the obstetrician should
consider the abdominal girth and the fetal tone ( ratio
between the fetus and fluid on palpation) as certain
gestational ages may make the fundus uteri at the same
point on mother’s abdomen e.g. fundal level of 36w, 38
week, and 40 weeks. The obstetrician clinical sence is
important in such situations.

C- ULTRASOUND FETAL BIOMETRY
• Not all scans during pregnancy serve the same purpose
• Early pregnancy scans are the most accurate for dating.
• Mid-trimester scans are used for anomaly assessment.
• 3rd trimester scans may assess Liquor volume & fetal weight
accurately but are useless for dating

- It is crucial that the mother is questioned if a scan
was done early in pregnancy If so, she should be
asked if the findings corresponded to the LMP
dates.
- In clinical practice, if the discrepancy between
menstrual & u/s dates is < 7 days, the LMP dates
should be employed (as long as the periods are
regular) If the discrepancy is more then; the EDD
should be based on ultrasonographic fetal
biometry.

EARLY IN PREGNANCY : [BEFORE 14 WEEKS)
• Crown-rump length (CRL) & Gestational sac diameter (GSD)
1- CRL:
•The CRL is the BEST measurement
Early in pregnancy.
•Accurate up to ONE day!
•All mothers should have this

Gestational sac diameter (GSD)
 With a TVS, a 2- to 3-mm gestational sac can usually
be seen by 5 weeks from the last menstrual period .
 A yolk sac is usually seen by 6 menstrual weeks, or by
the time the mean diameter of the sac has reached 10
mm
 A fetal pole with heart tones is typically seen by the
completion of 7 menstrual weeks

A very early, 3-mm mean diameter intrauterine gestational
sac at 5 weeks postmenstruation
typical yolk sac. The mean sac diameter of the gestational
sac is 10.6 mm. The length and the anterior-posterior
dimensions of the gestational sac are measured on this
sagittal image of this retroverted uterus

A early 7 mm fetal pole corresponding to 6 weeks 4 days menstrual weeks

- The BPD measurement before gestation week 20 predict gestation
age with an accuracy of ±7–11 days .
- The precision tolerance of BPD dimension decreases during
the third trimester of pregnancy .
The HC is less affected than BPD by head shape variations and the
presentation of the fetus, so HC is preferred as a more valuable
measurement in assessing gestational age after 20 weeks gestation.
Second and Third Trimesters: [head & Femur & abdomen
1- Head measurement : BPD & HC

Biparietal diameter (BPD) dimension: outer to inner (A) as in Scandivavian and outer to outer (B) as
in Germany.

2- Femur Length (FL):
-FL measurements include the
ossified portion of the diaphysis
and metaphysis.
-It has been identified that the
accuracy of FL in the predictions
of gestational age is 2.8 weeks
(2 SD) . With increasing
gestational age, the
accuracy decreases.

3- Abdominal Circumference (AC):
-AC is measured by the ellipse facility of ultrasound
equipment with the stomach bubble and a short
segment of the umbilical vein at the level of the portal
sinus visible.
- AC should not be used at all to determine gestational
age; however, it is one of the key dimensions to assess
intrauterine growth restriction (IUGR) and fetal
macrosomia

Abdominal circumference (AC) dimension.

HOW TO CALCULATE GESTATIONAL AGE

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