Evaluation of thalamus echogenicity by ultrasound as a marker of fetal lung maturity

doi:10.4236/ojog.2012.23056

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Open Journal of Obstetrics and Gynecology, 2012, 2, 270-275 OJOG

http://dx.doi.org/10.4236/ojog.2012.23056 Published Online September 2012 (http://www.SciRP.org/journal/ojog/)

Evaluation of thalamus echogenicity by ultrasound as

a marker of fetal lung maturity

Faris A. Rasheed1, Zahraa’ M. Al-Sattam2, Saad A. Hussain3*

1Department of Obstetrics and Gynecology, Al-Kindi Medical College, University of Baghdad, Baghdad, Iraq

2Department of Ob s tetrics, Al-Elwiya Maternity Teaching Hospital, Baghdad, Iraq

3Department of Pharmacology and Toxicology, College of Pharmacy, University of Baghdad, Baghdad, Iraq

Email: *[email protected]

Received 2 June 2012; revised 6 July 2012; accepted 18 July 2012

ABSTRACT

The present study evaluated fetal thalamic echogeni-

city by ultrasound as a possible marker of fetal lung

ma tur ity in comparison with other ultrasound maker s.

A prospective longitudinal study performed in Al-

Elwiya Maternity Teaching Hospital in Baghdad,

Iraq during the period from April 2010 to March

2011. One hundred and forty two pregnant women

(36 to 42 weeks of gestation) who were admitted for

elective cesarean section and referred for an obstetric

ultrasound scan at the same day of their elective ce-

sarean section were included. Scanning with linear

ultrasound with convex transducer frequency of 3.5

MHz was utilized to measure the biparietal diameter

and the state of echogenicity was recorded as echo-

genic or echolucent, in addition to amniotic fluid

vernix and the placental changes. The presence of

echogenic thalamus as a sign of fetal lung maturity

had a specificity of 86.53% which is higher than the

three other signs of lung maturity; the positive pre-

dictive value was (89.6%) which is also higher than

the three other signs, but the sensitivity was 63.33%

and negative predictive value was 57.69% which is

lower than the presence of vernix in the amniotic

fluid, 86.66 and 67.56 respectively. In conclusion,

evaluation of echogenic thalamus is beneficial, and

can be considered as a new marker of fetal lung

maturity; however, further studies are required to

strengthen such idea.

Keywords: Fetal Lung Maturity; Thalamic Echogenicity;

Ultrasound

1. INTRODUCTION

Fetal lung immaturity is a major problem in the mana-

gement of elective birth with respect to predicting the

development of infant respiratory distress syndrome

(IRDS) in the neonate after birth. Although gold-standard

measures of fetal lung immaturity were the chemical,

biological and physical properties of amniotic fluid [1],

the fluid was obtained by invasive technique through

amniocentesis. Moreover, lamellar body count was eva-

luated and found to be an easy, rapid and cost-effective

as a predictive measure fetal lung maturity [2]. Fetal lung

maturity was indirectly assessed by ultrasonic fetal lung

morphology [3], fetal respiratory movement or fetal na-

sal respiratory flow imaging by Doppler ultrasound [4],

ripening of the placental image [5], Doppler blood flow

wave forms [6] or the intrauterine amniotic fluid choline

by magnetic resonance spectroscopy [7]. Noninvasive

direct assessment of fetal lung maturity was studied by

the ultrasonic fetal lung frequency component [8], B-

mode image texture [9] and special mean gray level [10],

where the ultrasonic methods should rely on a special

computer system or the mean gray level was so unstable

to the device gain and the depth of the subjects th at every

measurement needed test piece calibration. Ultrasono-

graphy has contributed by the use of chest circumference

and area [11] or by comparison with other fetal measure-

ments [12]. Attempts have also been made to correlate

the ultrasonic appearances of the placenta with fetal lung

maturity. In all of these studies, however, the sensitivity

and positive predictive value were poor, although the

non-invasive nature of this approach and rapidity of eva-

luation were attractive [13]. Estimates of direct fetal lung

volume have been made using 3-D ultrasound [14], but

these methods may be affected by liquor volume and do

not provide any additional physiological information

relating to potential function. In an attempt to establish

further non-invasive method for evaluation of fetal lung

maturity, the present study was designed to evaluate the

validity of measuring fetal thalamic echogenicity by ul-

trasound as a marker of fetal lung maturity.

2. MATERIAL AND METHODS

2.1. Patients and Study Design

The present prospective longitudinal study involved 142

*Corresponding a uthor.

OPEN ACCESS

F. A. Rasheed et al. / Open Journal of Obstetrics and Gynecology 2 (2012) 270-275 271

pregnant women (age 31.0 ± 5.2 years) from 36 to 42

weeks of gestation (38.8 ± 2.6 weeks) who were admit-

ted for elective cesarean section for many indications

(Table 1) in Al-Elwiya Maternity Teaching Hospital/

Baghdad during the period from April 2010 to March

2011; after being fully familiarized with study protocol

and techniques; the protocol was in compliance with the

principles of the 1996 amendment the Declaration of

Helsinki, and the study was approved by both the Minis-

try of Health ethical committee and institutional review

board of Al-Kindy College of Medicine; the protocol

was compatible with the safety guidelines published by

the National Radiological Protection Board [15] and in-

formed written consent was obtained from each woman

prior to evaluation.

The inclusion criteria include women with single vi-

able pregnancy, uncomplicated pregnancy (hypertension

or diabetes), normal amount of liquor approved by ultra-

sound and Rh positive mothers. All included women

were examined by linear ultrasound with convex trans-

ducer frequency of 3.5 MHz (Braun, UK) at the Radio-

logy Department at the same day of their elective cesar-

ean section. During ultrasound examination we look for

the biparietal diameter (transthalamic plane) which was

measured in millimeters, and the state of echogenicity of

the thalamus was recorded as echogenic or echolucent in

comparison with the brain tissue between the thalamus

and the parietal bone which is echogenic through out

pregnancy; if the echogenicity of the thalamus appears

like brain tissue, it is considered echogenic, while if it

appears without echoes inside the thalamus it is echolu-

cent. The amniotic fluid particles (vernix) and the pla-

cental changes were also measured as a part of fetal

wellbeing assessment. All cases with complicated preg-

nancy, multiple pregnancy and pregnancy with known

fetal malformations, or those with oligohydramnios or

polyhydramnios were excluded.

2.2. Statistical Analysis

Statistical analysis was performed using the Statistical

Package for Social Sciences version 17.0 (SPSS Inc.,

Table 1. Indications of elective cesarean section.

Indications

Previous 1CS 38%

Previous 2 CS and more 23%

Primigravida 20%

Malpresentation 13%

For tubal ligation 3%

Postdate 3%

Chicago, IL, USA). The results of the study were ana-

lyzed statistically using descriptive statistics to describe

the observed frequencies with their percentages (Cross

tabulations). The validity of the US indicators compared

with fetal lung maturity were analyzed fo r validity of the

screening test through calculating sensitivity and speci-

ficity; the sensitivity is de fined as the probability of test-

ing positive if the disease is truly present and is calcu-

lated by: Sensitivity = (True positive by the test)/(True

positive + false negative) × 100%, while specificity is

defined as the probability of screening negative if the

disease is truly absent and is calculated by: Specificity =

(True negative by the test)/(True negative + false positive)

× 100% [16].

3. RESULTS

The changes in thalamic echogenicity in different gesta-

tional stages are demonstrated in Figures 1-4, which

represent the outcome in 64 cases (45.1%) of the evaluated

women.

Figure 1. Ultrasonograph shows echolucent fetal thalamus (T)

at 36 weeks of gestation (white arrows).

Figure 2. Ultrasonograph shows echolucent fetal thalamus (T)

at 36 weeks of gestation (white arrows).

F. A. Rasheed et al. / Open Journal of Obstetrics and Gynecology 2 (2012) 270-275

272

Table 2 showed that for the 4 ultrasonic criteria meas-

ured during the present study, the specificity and the

positive predictive values are higher for the thalamic

density. However, the pr esence of vernix in the amniotic

fluid showed more sensitivity (86.66 %) versus 63.33% in

the thalamic density. Meanwhile, Table 3 showed that

the four ultrasound parameters of lung maturity were in-

creased with gestational age, and the presence of vernix

in the amniotic fluid shows the highest and more predic-

tive percent compared to the others (73.9%), while the

feature of echogenic thalamus demonstrate relatively

lowest value (45.1%), though it appears comparable to

that of grade 2 and 3 placental calcification feature.

Concerning the neonatal outcome of the present study,

Table 4 showed that neonatal morbidity mostly occu rs at

early stages of the gestational age (less than 39 weeks)

and represents 35/52 of total admission to the Neonatal

Intensive Care Unit (NICU), and the admission was

19/52 for less than 24 hour and 5/52 for more than 48

hours. The present study did not report any neonatal

death up to discharge of ne w bo r n babi es fr o m the NICU.

Figure 3. Ultrasonograph shows echogenic thalamus (T) at 38

weeks of gestation (white arrows).

4. DISCUSSION

It would be convenient to predict fetal lung immaturity

before an elective birth noninvasively to allow thera-

peutic protection against possible respiratory distress

syndrome (RDS) in the neonate, or in some cases, to esti-

mate the effect of adrenal steroid treatment of a preterm

fetus by repeated tests that can be freely performed by

noninvasive techniques. Prenatal diagnosis permits deli-

very by planned caesarean section sufficiently early to

Figure 4. Ultrasonograph shows echogenic thalamus (T) at 38

weeks + 3 days of gestation (white arrows).

Table 2. Comparison between the sensitivity and the specificity of the four ultrasound parameters in our study as a sign of fetal lung

maturity.

Ultrasound parameters Sensitivity (%)Specificity (%) Positive predictive value (%) Negative predictive value (%)

Echogenic thalamus 63.33 86.53 89.6 57.69

Placental calcification grade 2 and 3 51.11 71.15 75.40 45.67

Biparietal diameter above 9.2 cm 62.22 42.30 65.11 39.28

Presence of amnio t ic fl u id ve rnix 86.66 48.07 74.28 67.56

Table 3. The predicted percent US outcome in the four ultrasound parameters of fetal lung maturity ranked according to gestational

age.

Gestational age

Ultrasound parameters 36 week 37 week 38 week 39 week 40 week 41 - 42 week Total

Number of cases 12 31 45 38 13 3 142

Echogenic thalamus 2 (16.7) 14 (45.2%)14 (31.1%)21 (55.3%)10 (76.9%) 3 (100%) 64 (45.1%)

Placental calcification grade 2 and 3 3 (25% ) 13 (41.4%)16 (35.6%)19 (50%) 8 (61.5%) 2 (66.7%) 61 (43%)

Biparietal diameter above 9.2 cm 1 (8.3%) 8 (25.8%)30 (66.7%)33 (86.8%)11 (84.6%) 3 (100%) 86 (60.6%)

Presence of amniotic fluid vernix 4 (33.3%) 22 (71%) 31 (68.9%)34 (89.5%)11 (84.6%) 3 (100%) 105 (73.9%)

OPEN ACCESS

F. A. Rasheed et al. / Open Journal of Obstetrics and Gynecology 2 (2012) 270-275 273

Table 4. Neonatal outcome according to evaluation of thalamic echogenicity.

Gestational age

Neonatal outcome 36 week37 week38 week39 week40 week 41 - 42 weekTotal

Number of cases 12 31 45 38 13 3 142

Good neonatal outcome

(not needed to be admitted to the NICU) 5 (41.6%)18 (58.1%)30 (66.7%)24 (63.2%)10 (76.9%) 3 (100%) 90 (63.4%)

Total admission to the NICU 7 (58.3%)13 (41.9%)15 (33.3%)14 (36.8%)3 (23.1%) 0 52 (36.6%)

Admission to the NICU less than 24 hrs 1 (8.3%)8 (25.8%)10 (22.2%)9 (23.7%)3 (23.1%) 0 31 (21.8%)

Admission to the NI CU from 24 to 48 hrs 4 (33.3%)4 (12.9%)3 (6.7%)5 (13.2%)0 0 16 (1 1.3%)

Admission to the NICU more than 48 hrs 2 (16.7%)1 (3.2%)2 (4.4%)0 0 0 5 (3.5%)

TTN or RDS 3 (25%) 10 (32.3%)13 (28.9%)14 (36.8%)3 (23.1%) 0 43 (30.3%)

RDS 4 (33.3%)3 (9.6%)2 (4.4%)0 0 0 9 (6.3%)

Treated hypoglycemia 1 (8.3%)1 (3.2%)2 (4.4%)0 0 0 4 (2.8%)

Neonatal mortality 0 0 0 0 0 0 0

NICU: Neonatal Intensive Car e Unit; TTN: Transient Tachypnea of the Newborn; RDS: Respir atory Distress Syndrome.

avoid the possible complications. A review of 155 cases

found that prenatal diagnosis results in a 95% reduction

in late fetal and neonatal mortality [17]. Such observa-

tions have inspired several clinical groups [17-20] and

parents to recommend routine identification of umbi-

lical cord insertion at 20 weeks of gestation and tar-

geted screening for vasa praevia in women identified as

at high risk for the condition. Many studies of ultra-

sound prediction of fetal lung maturity were used to

compare ultra-sound parameters with tests of amnio-

centesis to assess that sign with lung maturity [10].

Amnio centesis is not ava ilable in our prac tice, so in the

present study we compare the ultrasound parameters

with the neonatal outcomes (APGAR score, the need

for admission to the NICU and any sign of respiratory

distress). From clinical practice it has been noticed that

thalamic density increased with gestational age, so the

present study; for the best of our knowledge, was car-

ried out to assess this sign and try to adopt it as an

indicator for lung maturity; this subject was presented

for the first time and not previously reported elsewhere.

In the present study, through non-invasive technique,

we try to evaluate fetal thalamic echogenicity by ultra-

sound as a new indicator for lung maturity and compare

it with other previously known signs of fetal lung

maturity such as biparietal diameter, amniotic fluid

vernix and placental changes. The data showed that the

thalamus is echogenic in 64 cases (45%) and echolucent

in 78 cases (55%) and the echogenicity increases with

the increase in the gestational age; this result seems

comparable to a previously reported data that correlate

thalamic echogenicity with gestational age by the same

author [21]. The presence of echogenic thalamus as a

sign of fetal lung maturity by ultrasound was found to

be of high specificity in detecting fetal lung maturity

and demonstrates high positive predictive value com-

pared to other ultrasound parameters utilized in the

present study; however, the sensitivity and negative

predic tive value w ere reported to be lower compared to

the signs of vernix in the amniotic fluid. Although the

utilized technique is simple and can be easily performed

in all obstetrics units, the echo reflection is not meas-

urable and depends on the experience of the examiner,

which represents a major difficulty in interpretation of

data.

It has been reported that both grade II and III placenta

predicted fetal lung maturity in 100% of cases [22],

suggesting that placental grading could replace estima-

tion of the L/S ratio and thereby avoid amniocentesis and

its complications. However, subsequent workers in this

field have cast doubt on the reliability of placental grad-

ing as a predictor of fetal lung maturity [23,24], and the

subject has became controversial for a many reasons

including presence of complications like hypertension,

diabetes mellitus or Rh iso-immune disease. Accordingly,

the search for novel signs and markers for prediction of

lung maturity became an interested discipline in the

obstetrics field. The present study confirmed the previ-

ously reported data that grades II and III placentas were

highly associated with fetal lung maturity [25]. Mean-

while, the finding of fetal lung maturation in 83.3% of

infants in grade III group justifies using this group to

indicate fetal lung maturity before repeat caesarean

section in woman with reasonable certainty to reach 38

weeks of gestation and has no complicating diseases.

However, because only 18% of our patients had a grade

F. A. Rasheed et al. / Open Journal of Obstetrics and Gynecology 2 (2012) 270-275

274

III placenta, those with grade II placenta satisfying the

provisos mentioned above should be managed in the

same way as those in grade III on the basis of our find-

ings, and even with this group 75.4% of fetuses had

achieved lung maturity, such finding seems compatible

with that reported previously [22]. Hadlock et al. (1985)

utilized amniocentesis for determination of fetal lung

maturity and ultrasonographic (US) evaluation of the

biparietal diameter (BPD) and placental grade were per-

formed simultaneously and concluded that the ability of

the sonograph ic parameters to predict fetal lung maturity

was closely related to menstrual age and the best use of

US for predicting fetal lung maturity is in establishing

menstrual age early in pregnancy [26]; the results ob-

tained in the present study support the previously men-

tioned idea. Concerning evaluation of the presence of

FFPs in the amniotic fluid, Gross et al. (1985) correlate

this ultrasound finding with fetal lung maturity and

suggest that presence of FFPs on real-time ultrasound

could be used to confirm fetal lung maturity [27]. The

data presented in this study support the previously men-

tioned one an d more than on e u ltr asound mar k er s may be

better correlated for prediction of fetal lung maturity.

Moreover, the present study demonstrates that a BPD

greater than or equal to 92 mm predicted 56 of 90

(62.22%) cases with mature lung, while the presence of

FFPs predicted 78 of 90 (86.66%); when utilized toge-

ther, a BPD equal to or greater than 92mm and/or the

presence of FFPs predicted 83 of 90 (92.22%) of cases

with mature fetal lung, and such finding complies with

data reported by others, where BPD greater than or equal

to 92 mm and the presence of free-floating particles are

independent, and considered as complimentary predic-

tors of fetal lung maturity [28]. Subjective approach

during measurement of echogenicity and lack of a me-

thod to measure the gain of echoes are the limitations of

the present study, in hope th at we can overco me in future

studies. In conclusion, evaluation of echogenic thalamus

by ultrasound at the lev el of the BPD is of value, and can

be considered as a new marker of fetal lung maturity;

however, further studies are required to strengthen such

idea.

5. ACKNOWLEDGEMENTS

The authors would like to thank University of Baghdad for support;

also they gratefully acknowledge the ultrasonography staff of Al-

Elwiya Maternity Teaching Hospital for technical assistance.

REFERENCES

[1] American College of Obstetricians and Gynecologists.

(2008) ACOG practice bulletin No. 97. Fetal lung ma-

turity. Obstetrics and Gynecology, 11 2, 717-726.

doi:10.1097/AOG.0b013e318188d1c2

[2] Bahadsari, S. and Changizi, N. (2005) Association be-

tween lamellar body count and respiratory distress in

neonates. Saudi M ed ic al Journal, 26, 1414-1146.

[3] Gerads, F.A., Twisk, J.W., Fetter, W.P., Wijnaendts, L.C.

and van Vugt, J.M. (2008) Predicting pulmonary hy-

poplasia with 2- or 3-dimensional ultrasonography in

complicated pregnancies. American Journal of Obstetrics

and Gyneology, 198, 140.

[4] La Torre, R., Cosmi, E., Anceschi, M.H., Piazze, J.J.,

Piga, M.D. and Cosmi, E.V. (2003) Preliminary report on

a new and noninvasive method for the assessment of fetal

lung maturity. Journal of Perinatal Medicine, 31, 431-

434. doi:10.1515/JPM.2003.067

[5] Shan, A.G. and Graham, D. (1986) Relationship of pla-

cental grade to fetal pulmonary maturity and respiratory

distress syndrome. American Journal of Perinatology, 3,

53-55. doi:10.1055/s-2007-999827

[6] Piazze, J.J., Ancesc hi, M.M., Picone, G., Cereskja, A., La

Torre, R. and Cosmi, E.V. (2003) Association between

maternal-fetal Doppler velocimetry and fetal lung ma-

turity. Journal of Perinatal Medicine, 31, 484-488.

doi:10.1515/JPM.2003.074

[7] Clifton, M.S. , Joe, B.N., Zekt ozer, A.S., Kurhanewicz , J.,

Vigneron, D.B., Coakley, F.V., et al. (2006) Feasibility of

magnetic resonance spectroscopy for evaluating fetal

lung maturity. Journal of Pediatric Surgery, 41, 768-773.

doi:10.1016/j.jpedsurg.2006.02.020

[8] Sohn, C., Stolz, W. and Bastert, G. (1991) Diagnosis of

fetal lung maturity by ultrasound: A new method and ﬁrst

results. Ultrasound in Obstetrics and Gynecology, 1, 345-

348. doi:10.1046/j.1469-0705.1991.01050345.x

[9] Prakash, K.N., Ramarkrishnan, A.G., Suresh, S. and

Chow, T.W. (2001) An investigation into the feasibility of

fetal lung maturity prediction using statistical textural

features. Ultrasonography and Imaging, 23, 39-54.

[10] Takesin, I., Anderer, G., Hellmeyer, L., Stein, W., Kunert,

M. and Schmidt, S. (2004) Assessment of fetal lung de-

velopment by quantitative ultrasonic tissue characteriza-

tion: A methodical study. Prenatal Diagnnosis, 24, 671-

676. doi:10.1002/pd.951

[11] Nimrod, C., Davies, D., Iwanicki, S., Harder, J., Prsaud,

D. and Nicholson, S. (1986) Ultrasound prediction of

pulmonary hypoplasia. Obstetrics and Gynecology, 68,

495-498.

[12] Johnson, A., Callan, N., Bhutan, V.K., Colmorgen, G.H.C.,

Weiner, S. and Bolognese, R.J. (1987) Ultrasonic ratio of

fetal thoracic to abdominal circumference: An association

with pulmonary hypoplasia. American Journal of Obstet-

rics and Gynecology, 157, 764-769.

[13] Kazzi, G.M., Gross, T.L., Sokol, R.J. and Kazzi, S.N.

(1985) Non-invasive prediction of HMD: An optimised

classification of sonographic placental maturation. Ame-

rican Journal of Obstetrics and Gynecology, 152, 213.

[14] Lee, A., Kratochwil, A., Stumpflen, I., Deutinger, J. and

Bernaschek, G. (1996) Fetal lung volume determination

by three-dimensional ultrasonography. American Journal

of Obstetrics and Gynecology, 175, 588-592.

F. A. Rasheed et al. / Open Journal of Obstetrics and Gynecology 2 (2012) 270-275

275

OPEN ACCESS

doi:10.1053/ob.1996.v175.a72478

[15] Mole, R.H. (1993) The biology and radiobiology of in

utero development in relation to radiological protection.

British Journal of Radiology, 66, 833-841.

doi:10.1259/0007-1285-66-792-1095

[16] Beth, D. and Robert, G. (2004) Basic and clinical biosta-

tistics, 4th Editon, McGraw-Hill, New York.

[17] Oyelese, Y., Catanzarite, V., Prefumo, F., Lashley, S.,

Schachter, M., Tovbin, Y., et al. (2004) Vasa previa: The

impact of prenatal diagnosis on outcomes. Obstetrics and

Gynecology, 103, 937-942.

doi:10.1097/01.AOG.0000123245.48645.98

[18] Baulies, S., Maiz, N., Munoz, A., Torrents, M., Echevar-

ria, M. and Serra, B. (2007) Prenatal ultrasound diagnosis

of vasa praevia and analysis of risk factors. Prenatal

Diagnosis, 27, 595-599. doi:10.1002/pd.1753

[19] Gandhi, M., Cleary-Goldman, J., Ferrara, L., Ciorica, D.,

Saltzman, D. and Rebarber, A. (2008) The association

between vasa previa, multiple gestations, and assisted re-

productive technology. American Journal of Perinatology,

25, 587-589. doi:10.1055/s-0028-1090584

[20] Sinha, P., Kaushik, S., Kuruba, N. and Beweley, S. (2008)

Vasa praevia: A missed diagnosis. Journal of Obstetrics

and Gynecology, 28, 600-603.

doi:10.1080/01443610802344365

[21] Ras he ed, F.A. (2001) Fetal thalamus echog e ni ci ty c h an ge s

with increasing fetal age. Iraqi Medical Journal, 50,

20-24.

[22] Shweni, P.M. and Moodley, S.C. (1986) Placental grading

by ultrasonography as an index of fetal maturity, its ap-

plication to the problem of elective caesarean section.

South African Medical Journal, 10, 525-528.

[23] Kazzi, G.M., Gross, T.L., Rosen, M.G. and Jaatoul-Kazzi,

N.Y. (1984) The relationship of placental grade, fetal lung

maturity and neonatal outcome in normal and compli-

cated pregnancies. American Journal of Obstetrics and

Gynecology, 148, 54-58.

[24] Clair, M.R., Rosenberg, E., Tempkin, O., Andreoni, R.F.

and Bowie, J.D. (1983) Placental grading in the compli-

cated or high-risk pregnancy. Ultrasound Medicine, 2,

297-301.

[25] Loret de Mola, J.R., Judge, N., Entsminger, C., DeViney,

M., Muise, K.L. and Duchon, M.A. (1998) Indirect pre-

diction of fetal lung maturity. Value of ultrasonographic

colonic and placental grading. Journal of Reproductive

Medicine, 43, 898-902.

[26] Hadlock, F.P., Irwin, J.F., Roecker, E., Shah, Y.P., Deter,

R.I. and Rossavik, I.K. (1985) Ultrasound prediction of

fetal lung maturity. Radiology, 155, 469-472.

[27] Gross, T.L., Wolfson, R.N. and Sokol, R.J. (1985) Sono-

graphically detected free-floating particles in amniotic

fluid predict a mature lecithin—Sphingomyelin ratio.

Journal Clinical Ultrasound, 13, 405-40 9.

doi:10.1002/jcu.1870130606

[28] Mullin, T.J., Gross, T.L. and Wolfson, R.N. (1985) Ultra-

sound screening for free-floating particles and fetal lung

maturity. Obstetrics and Gynecology, 66, 50-54.