Mid sagittal section of the fetus in neutral position Sri Lanka College of Obstetricians and Gynaecologists Guideline

Around 370,000 babies are born each year in Sri Lanka. There is no national policy of ultrasound assessment of gestational age for all pregnancies. Currently, last regular menstrual period (LRMP) is used for calculation of the gestational age. In case of ultrasound dating, timing of ultrasound will vary upon the preference of the Obstetrician. The aim of this guideline is to set evidence-based standards in gestational age assessment and determination of chorionicity in multiple gestations.


Aims
Around 370,000 babies are born each year in Sri Lanka.There is no national policy of ultrasound assessment of gestational age for all pregnancies.Currently, last regular menstrual period (LRMP) is used for calculation of the gestational age.In case of ultrasound dating, timing of ultrasound will vary upon the preference of the Obstetrician.The aim of this guideline is to set evidence-based standards in gestational age assessment and determination of chorionicity in multiple gestations.

Background and introduction
Gestational age assessment by menstrual history is not accurate because up to 40% of women are uncertain of their menstrual dates or ovulation may not exactly correspond with the mid menstrual cycle 1 .Therefore, gestational age derived from last menstrual period is subjected to both random and systematic errors 2 .Overestimation of true gestational age by menstrual history increases the prevalence of post date pregnancies 3 .First trimester ultrasound dating using crownrump length (CRL) has proven to be more reliable than methods based on LRMP to predict the date of delivery.First trimester ultra-sound dating can reduce the percentage of post term pregnancies by up to 60% 4,5,6 .Second trimester pregnancy dating is also reliable but it is not as accurate as first trimester dating 6 .Further, early determination of chorionicity is important in order to optimize the outcome in twin pregnancies.

Identification and assessment of evidence
This guideline was developed after searching evidence from EMBASE, Medline and PubMed electronic databases.Relevant randomized controlled trials, systematic reviews, meta-analyses and cohort studies were considered.Technique for CRL CRL measurements can be obtained either transabdominally or transvaginally.The whole fetus should be horizontal on the screen so that the line between crown and rump is at 90° to the ultrasound beam and following criteria should be met (Figure 1).• A long axis of the fetus in midsagittal (median) section is obtained.
• A fetus should be in neutral position • Measurements are taken from The top of the head (crown) to the end of the trunk (rump) using the onscreen calipers.

What is the best method of determining gestational age during second trimester?
Pregnant women who present at or beyond 14 weeks of gestation up to 26 weeks should be offered an ultrasound scan to estimate GA using head circumference (HC) 9 .HC measurement can be calculated from the biparietal diameter (BPD) and the occipital-frontal diameter (OFD) using the formula.
Modern ultrasound machines can calculate the HC directly from the diameters of the head using the ellipse facility.Deriving the head circumference in this way is acceptable provided that the above equation is used.Gestational age should be estimated from HC using the Chitty's formula 10 .

Technique for HC
The image should be frozen when the following landmarks are identified (Figure 2).
• A cross-sectional view of the fetal head at the level of the ventricles should be obtained.
• A rugby football-shaped skull, rounded at the back (occiput) and more pointed at the front (synciput).
• A long midline equidistant from the proximal and distal skull echoes.
• The cavum septum pellucidum bisecting the midline one-third of the distance from the synciput to the occiput.
• The two anterior horns of the lateral ventricles symmetrically placed about the midline.
• All or part of the posterior horns of the lateral ventricles symmetrically placed about the midline.
• To measure the BPD and OFD the intersection of the calipers should be placed on the outer border of the skull ('outer to outer').

What is the best method of determining gestational age for late bookers (beyond 26 weeks)?
After 26 weeks gestation, it is not possible to assess GA accurately using ultrasound.Pregnant women who present at that time should be informed of the limitations of ultrasound dating and be offered two scans 2 weeks apart.A tentative estimate of gestational age should be assigned using HC during the first scan and repeat HC measurement in 2 weeks should be arranged in order to confirm the GA.Gestational age can be confirmed if the HC in second scan corresponds with the initial ultrasound dating 11 .If they do not tally, senior opinion should be sought.

What is the best method of determining gestational age in multiple pregnancies?
Women with twin and triplet pregnancies must be offered a first trimester ultrasound scan when crown-rump length measures between 45 to 84 mm (at approximately 11 +0 -13+ 6 ) to estimate gestational age and to determine chorionicity 12,13 .Women with multiple pregnancies who present at or beyond 14 weeks of gestation should be offered an ultrasound scan to estimate gestational age using head circumference (HC) 14 .Measurement from the largest baby is used, to avoid the risk of estimating it from a baby with early growth pathology 12,14,15 .Chorionicity must be determined during the first trimester by ultrasound using the number of placental masses, the lambda or T-sign (Figure 3) 13 .
If a woman with a twin or triplet pregnancy presents after 14 +0 , chorionicity is determined at the earliest opportunity using one or combination of the following ultrasound markers [16][17][18] .
o If it is not possible to determine chorionicity by ultrasound at the time of detecting the twin or triplet pregnancy, a second opinion should be sought from a senior doctor or refer the woman to an expert who is competent in determining chorionicity as soon as possible.
If it is difficult to determine chorionicity, even after referral (for example, because the woman has booked late in pregnancy), the pregnancy should be managed as monochorionic until proved otherwise.
The relative orientation of the fetuses to each other in relation to long axis of the uterus (Figure 4) should be defined as either lateral (left/right) or vertical (top/bottom).Lateral fetal orientation is associated with an intertwin membrane running vertically along the longitudinal axis of the uterus and vertical fetal orientation is associated with an intertwin membrane running horizontally across the longitudinal axis of the uterus.Assigned nomenclature to babies (for example, top/bottom, or left/ right) in twin and triplet pregnancies should be documented clearly in the woman's notes to ensure consistency throughout pregnancy 15 .

Figure 1 .
Figure 1.Mid sagittal section of the fetus in neutral position

Figure 2 . 8 .
Figure 2. Transverse section of the fetal head demonstrating the landmarks required to measure the HC using the lateral ventricles view.CSP: cavum septum pellucidum, PH: Posterior horn of the lateral ventricle The number of placental masses o The lambda or T-sign o Membrane thickness (thresholds for determining monochorionicity is from 1.0 to 2.0 mm) o Discordant fetal sex.

Figure 4 .Figure 3 .
Figure 4. Diagrammatic representation of twin orientation relative to the longitudinal axis of the uterus.The twins may have a top/bottom (T/B) (vertical) (a) or right/left (R/L) (lateral) (b) orientation

4. What is the best method of determining gestational age during first trimester?
Ideally, scans should be performed between 11 +0 and 13 +6 weeks and use measurement of crownrump length (CRL) (CRL measures from 45 mm to 84 mm) to determine gestational age.Robinson's formula should be GA = 8.052 × (CRL × 1.037) 1/2 + 23.73