determining gestational using different techniques

1. How to determine gestational age of
pregnancy for each stage, advantages and
disadvantages of each methodology
Sasha Bondi Bsc Nursing Student
UZCHS Nursing Department

2. DEFINITION
Gestation is a term that describes the time between conception and birth during which many complex
developments take place and a baby grows and develops in the mothers uterus.
What is gestational age?
• usual way for describing the age of pregnancy, how far along it is.
• Usually expressed as a combination of weeks and days,
• gestational age counts from the first day the mothers last menstrual
period to the present, so
• it technically includes about 2 weeks during which the woman was
not pregnant.
• Gestational age is different from foetal age, which is the number of
weeks that have passed since conception.

3. Methods of estimating gestational age
Ultra sound
• It is feasible in a majority pregnancies.
• It is feasible in a majority pregnancies.
• It uses different methods to estimate gestational age i.e
 Gestational mean sac diameter
 Bi-parietal diameter
 Head circumference
 Abdominal circumference
 Femur length

4. Gestational Mean Sac Diameter
• Used in 1st trimester
• identified by Trans-abdominal ultrasound as early as 5 weeks’ gestation and may be seen as early as 4 weeks’
gestation by transvaginal ultrasound.
• gestational sac is the large cavity of fluid surrounding the embryo during embryogenesis
• consists of the extra-embryonic coelom, also called chorionic cavity.
• It is the only available structure that can be used to determine if an intrauterine pregnancy exists until the embryo
is identified.
• first imaged in the longitudinal plane, obtaining long axis and anteroposterior measurement, width
measurement.
• The three measurements are averaged to obtain the gestational sac mean diameter
• accuracy of gestational mean diameter as a measure of gestational age was found to be +/- 1week.
• Mathematical formula to calculate gestational age of pregnancy 28days (4weeks) + (mm × days). 1mm
corresponds to 1 day e.g. gestational sac measuring 11mm would approximately be
• 28days (4 weeks) + 11 days= 39days (5weeks 4days)

5. Advantages
• In experienced hands, may detect pregnancy as early as 30 days using
TVU
• Is a positive marker that woman is pregnant
• Allows to detect if pregnancy will be normal or a gestational
miscarriage has occurred, early
•

6. Crown-Rump Length (CRL)
• involves measurement of foetal length from tip of cephalic pole to the tip of the
caudal pole.
• Measurement of the embryo usually identifiable at 6 to 7 weeks’ gestation.
• accurately date pregnancy between 7-13 weeks’ gestation.
• After 12 weeks’ gestation excessive curvature of the foetus may lead to
erroneous shortening of CRL measurement.
• Accuracy of CRL is within +/- 5-7 days.
• considered one of the more accurate methods of estimating gestational age in
first trimester. Mathematical formula:
6 weeks + (CRL×days) this relies on the normal growth of the foetus of 1 mm a
day after the 6th week e.g. CRL of 16mm would correspond to 42 days(6weeks) +
(16×1)days = 58days (8weeks and 2days)

8. Bi-parietal diameter
• It’s the diameter between the 2 sides of the foetal head.
• This is measure after 13 weeks.
• Between 12 to 26 weeks’ gestation the BPD is accurate to within +/-10-
11days.
• 2 weeks into 2nd trimester accuracy of BPD progressively decreases and by
term is +/-3weeks.
• This method is more accurate when the foetal head is ovoid.
• If head is unusually round (bradycephalic) or unusually elongated
(dolichocephalic) BPD would overestimate or underestimate gestational age
respectively.

9. Head circumference
• Measures circumference of the foetal head.
• Usually done after 13 weeks of gestation.
• Ultrasonographers uses the machine to measure by track and ball on the
ultrasonic equipment.

10. Abdominal circumference
• Measured at the level of the foetal liver, most important
measurement to make in late pregnancy.
• It reflects foetal size and weight rather than age.
• For age it is in accurate. It should not be used to date gestational age

11. Femur length
• Largest of the long bones and easiest to image,
• can be adequately visualized from 14 weeks’ gestational age until
delivery.
• Measured along the long axis of the bone.
• The length can be accurately used to predict gestational age between
14 weeks and term.
• Biological variation can lead to inaccuracies.

13. Symphysis fundal height
• This is much more accurate than weighing the mother
• predominant method for estimating gestational age in resource poor
nations.
• Measurement of the fundal height will give u three thing;
gestation age,
EDD and confirming EDD if LNMP is available
and if the foetus is growing well and at the expected rate
with fundal height there are two methods; the finger method and
measuring with a soft tape measure.

14. Finger Method
• With finger mehtod, when the top of the uterus is below the umbilicus, note
how many fingers below it is
• if it’s above measure how many fingers above the umbilicus.
• This method is only applicable between 15-25 weeks
• recording we put a +sign in front how many fingers if the fundus is above the
umbilicus or a – sign for fingers below.
• The finger method is not very precise,
• even if the same observer measured the same woman twice on the same day
there might be variations (intra-observer variation)
• the finger method assumes the distance between the symphis pubis and the
umbilicus is 20 cm at 20 weeks gestation,
• Does not taking into account the difference from individual to individual, twins,
polyhydramnios, oligohydramnios and population characteristics
•

17. Measuring with soft tape measure
• tape measure is placed on the symphis pubis at one end and at the
fundus of the uterus at the other
• midwife faces the non-centimetre side to avoid bias.
• When the fundus is above the umbilicus the number of centimetres is
said to correlate with the number of weeks e.g. 26cm=26weeks.

20. Last Menstrual Period (Naegle’s Rule)
• It estimates a woman due date and gestational age based on the
woman’s last menstrual period (LMP).
• It bases its calculation that the woman has a 28 day menstrual cycle
• and that the whole gestational period will be 280 days (40weeks).
• This varies slightly because gestation might be slightly longer for first
time mothers.
•

21. How to use LMP (Naegle’s Rule)
• Subtract 3 months from the LMP
• Add 7 days to the LMP
• Add 1 year
• Use this calculation for LMPs between April to December
•
• Or
• Add 7 days to LMP
• Then add 9 months
• For patients whose LMP falls between January – March
• Examples
• LMP January 5, 2017
• + 7 days = January 12, 2017
• + 9 months = EDD: October 12, 2017
•
• LMP September, 28 2016
• -3months= June 28, 2016
• +7days= July 5, 2016
• +1 year =EDD: July 5, 2017
• This rule is not very accurate as not every woman knows her LMP, some women have 21 day or 35 day menstrual periods.

22. Advantages and Disadvantages
• Ultrasound allows early detection of pregnancy and early estimation of EDD.
• Can detect if woman is actually pregnant early, in the hands of an expert as early
as 30 days
• Is more accurate than LMP and measurement of symphysis fundal height
• With some of the techniques the foetus has to be in a certain position or the
head has to be of a certain shape limiting it use
• Foetal growth patterns not taken into account, maternal age, parity, pregnancy
maternal weight, geographic location and population characteristics.
• technical factors including interobserver error, different techniques of
measurements between different observers.

24. continued
• Discrepancies to consider for fundal height
• Fundal height may drop between visits.
• If the foetus turns sideways, as often occurs mid-pregnancy, the fundal height
can be usually shorter than expected.
• A breech baby, usually sitting up high in the uterus, will yield an unusually large
fundal height.
• Twins or more will give a fundal height way too big for the gestational age
• Oligohydramnios- too little fluid which could lead to a decreased fundal height.
• Polyhydramnios- too much fluid, leading an increased fundal height.
• Abnormal positions of the foetus close to term also makes the reading
inaccurate. In developing countries a lot of the women visit the antenatal clinic
1-2 weeks b4 term.

25. continued
• With LMP
• bases its calculation that the woman has a 28 day menstrual cycle
• Doesn’t take into account that every woman varies
• This rule is not very accurate as not every woman knows her LMP,
• some women have 21 day or 35 day menstrual periods.

27. References
• Baltzer FR et al: AMJ Obstet Gynecol, 146:973, 1983
• Campbell S, Warsof SL, Little D, Cooper Dj: Routine ultrasound
screening for the prediction of gestational age. Obstet Gynecol
65:613, 1985
• Macgregor S, Sabbagah R, Glob.libr.women’smed, (ISSN:1756-2228)
2008; DOI 10.3843/GLOWM.10206
• www.open.edu Antenatal care module:10mEstimating Gestational
Age from Fundal Height Measurement. Retrieved 15-9-2018