Page 1 of 1: USG in obs maturity of fetus

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ULTRASONOGRAPHY

Menstrual age: calculated from the 1st day of the Last Normal Menstrual Period.

A)The Period of zygote: The levels of subsequent development of nidation begins from the day of ovulation till 2 wks i.e. menstrual age of 4 wks.

B)Embryonic period:begins from the 3rd wk of ovulation till 8 wks i.e. from menstrual age of 5 wks till end of 8 wks.

C)Fetal period:starts from menstrual age of 8 to 10 wks and onwards.

Ultrasound is a noninvasive procedure.

USG IN THE 1ST TRIMESTER
Esp. TVS ( if values are obtained in TAS then deduct 1 week from each value)

1) Gestational Sac( gest ring):
It’s presence, number , size(dia), volume, location( within or outside uterus), site within uterus.
It is followed serially by monitoring the changes (increase/decrease) etc throughout pregnancy.
Gestational sac is normally visualized as a fine circular structure
-yolk sac –within the gest sac of the uterus
-echoes from the fetal pole/fetal no. during the period of the zygote
-embryo/ fetus within the gest sac
-length of fetal pole or early/late embryo or fetus e.g.
-Crown Rump length CRL from Vx to coccyx tip: because of the variability in the length of the legs and the difficulty of maintaining them in extension—measurement of Sitting Height (CRL) of the fetus is more accurate than that of the Standing Height( i.e. CrownHeel length) i.e, CH ( not HC which means Head Circumference)
Then onwards from the mid first trimester to late first trimester due to the dev of embryo and subsequently fetus ---we measure
-fetal Head Circumference(HC)
-ratio of volume of gest sac to fetal CRL
-small
-normal
-increased
-fetal Abdominal Circumference-at the level of umbilical vein( ductus venosus)
-fetal BPD (ultrasonic biparietal cephalometry)
-fetal Femur Length (FL)
-amniotic fluid volume
-fetal Humerus Length(HL)
-the fetal Abd Circumference/fetal Head Circumference ratio is normally 1:1 upto 35-36 wk
-thoracic dia and BPD ratio
-length of clavicle/ulna/tibia/fibula
-presence of fetal life detected by Doppler USG e.g. by detecting fetal activity such as:
1-fetal cardiac activity
2-fetal breathing activity
3-active fetal movts of the body and the limbs in later 1st trimester-TM e.g. fetal body motions such as fetal limb motions
-no of fetuses/ placental location
-associated any congenital malformations in early pregnancy( 6 wks) e.g. anencephaly and omphalocele
-rule out H mole/ectopic gest/simultaneous presence of IUCD in uterus /twins
-any associated adnexal masses/uterine masses
With increase in gest age observation becomes more clearly defined and can be visualized with certainty.
Estimation of gest age by USG is more accurate in early pregnancy( 1st TM) if records are available..esp..BPD and FL and CRL---as compared to those in Last Trimester.

Estimation of gest age is important: because an infant born at 36 wks when provided with the Best Extra uterine environment can survive in the Neonatal period.
A baby born at 40 wks can survive Neonatal period 99.9 %.

MIDPREGNANCY USG
1) Fetal BPD/AC
2) Fetal cardiac activity
3) Measurement of Amniotic Fluid Volume
-normal
-oligohydramnios
-hydramnios
4) Estimation of fetal weight—using fetal abd circumference and fetal BPD ratio
5) Placenta –no and site
6) Fetal Malformation
7) Fetal BPD and FL ratio and HC/AC ratio
8) Associated uterine/adnexal masses
LATE PREGNANCY USG
1) Fetal BPD and HC (are two different things) / No of fetuses and placentae / fetal cardiac activity
2) Lie, presentation, position of fetus
3) Location and grading of placentae and Am Fluid vol (vertical length of each pocket = < 2 cm and AFI = 5 cm-8 cm normally)
4) HC/AC/FL/HL/Fetal Thorax
5) Fetal weight is estimated by using indirectly-fetal abd circumference /FL/BPD
6) Umbilical cord study
7) Fetal malformations/twins
8) Associated uterine / adnexal masses
Serial USG findings are more important. There are variations in these values

Menst Age Gest Sac CRL F Head F Trunk F Weight
5 wk 5 mm 5 mm
7 wk 30 mm 10 mm
8 wk 45 mm 17 mm 10 mm 1 gm
9 wk 22 mm
10 wk 70 mm 32 mm 14 mm
11 wk 42 mm
12 wk 55 mm 21 mm 18 mm 1.4 gm
16 wk 102 mm 35 mm 30 mm 100-120 gm
24 wk 21 cm 600-630 gm or more
36 wk ≥32 cm(CHL) ≥47 cm ≥2500 gm
40 wk 36 cm 3405 gm or atleast 33 gm at 40 wk

Note - 1 kg =2.2 lbs
5½ lbs =2500 gms
BPD Gest Age At 36 wks occipitofrontal dia ≥11.5 cm
Fetal wt ≥2500 gm
Head Circumference≥33 cm
CRL ≥32 cm
CHL ≥47 cm
Thorax Circumference ≥30 cm
? 1-2 cm 10-12 wk
2.0 cm 12-14 wk
3.4 cm 16 wk
4.6 cm 20-24 wk
5.6 cm 24 wk
6.7 cm 26 wk
7.4 cm 28 wk
8.6-8.9 cm 36 wk
9.5 cm 40 wk
>9.6 cm-9.8 cm >40 wk
10.0 cm Definitely > 40 wk
Gest Age HC cm AC cm FL cm Thoracic Circumference
12w 6.8 4.6 0.7
20w 18.3 15.6 3.4
24w 23.0 20.2 4.5
36w 33.0 32.3 7.1 30
40w 34.6 35.3 7.7

Mean Value of HC/AC Gestational age
1.13 28w
1.04 34w
1.02 36w
0.99 38w
0.97 40w

• An USG performed earliest in gestation is regarded as the most accurate predictor of gestational age. After doing USG, fetal ultrasonic parameters e.g. BPD/HC/AC/FL of a given patient are compared to a sonogram chart previously standardised on a welldated pregnancy population. These charts are available for bedside comparison.
• There are variations within 1 to 2 weeks, but that is a minor problem.
• In doubtful cases repeat USG is done and other fetal maturity tests are done.

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