1
CORRESPONDENCE Algorithms for combining menstrual estimates of gestational age Sir, Blondel et al. 1 have demonstrated the apparent effect on rates of preterm and postterm birth rates when changing from menstrual to early ultrasound dating. Although confidence intervals for the differences in the results of the two methods is not provided in this paper, the large database of over 44,000 is likely to make the confidence intervals very narrow. The authors state that the ultrasound dating was based on the fetal biparietal diameter usually carried out between 16 and 18 weeks but do not tell us which formula was used. No doubt this could have varied between the two centres and over the time of the study. Two commonly used published formulae (Hadlock et al. 2 and Altman and Chitty 3 ) differ by 3.5 days for a biparietal diameter measurement of 41 mm (approximately equivalent to 18 weeks of gestation). We have carried out an audit of ultrasound dating 4 in a singleton pregnancy population restricted to those pregnancies with spontaneous onset of labour and delivery of a live healthy infant. In this audit, dating was also based on biparietal diameter, usually between 18 and 20 weeks. The audit has shown that the preterm rate can vary between 1.8% 1 and 3.7% 3 according to the two formulae referred above. This represents a 51% difference. This is another factor that needs to be taken into account when interpreting epidemiological reports on the changes in preterm and postterm births and we recommend that authors publish which dating formula has been used. References 1. Blondel B, Morin I, Platt RW, Kramer MS, Usher R, Breart G. Algorithms for combining menstrual estimates of gestational age: consequences for rates of preterm and postterm birth. Br J Obstet Gynaecol 2002;109:718 – 720. 2. Hadlock FP, Deter RL, Harrist RB, Park SK. Fetal biparietal diameter: a critical re-evaluation of the relationship to menstrual age by means of real time ultrasound. J Ultrasound Med 1982;1:97 – 104. 3. Altman DG, Chitty LS. New charts for ultrasound dating of pregnancy. Ultrasound Obstet Gynecol 1997;10:174 – 191. 4. Hutchon DJR. Publishing raw data and real time statistical analysis on e-journals, http://bmj.com/cgi/content/full/322/7285/530. BMJ 2001; 322:530. David J. R. Hutchon Darlington Memorial Hospital, Darlington, UK PII:S1470-0328(03)02833-7 Prenatal diagnosis of the Wolf–Parkinson–White syndrome by fetal magnetocardiography Sir, Khler et al. 1 are to be congratulated on their elegant use of fetal magnetocardiography to detect fetal Wolf– Parkinson – White syndrome. They tell us that this is clinically useful because digoxin can prove fatal in supraventricular tachycardias as a result of Wolf – Parkinson – White syndrome, and they therefore pre- scribed flecainide. Hopefully, we are not being too pedantic to suggest that this has no relevance whatsoever in the fetus, neonate or small child up to the age of around one year. Digoxin is certainly contra- indicated in adults with Wolf – Parkinson – White syndrome where the supraventricular tachycardia may be atrial fibrillation. This is because digoxin paradoxically accelerates conduction in the accessory bundle of Kent, and can cause a fatal circus rhythm. However, atrial fibrillation does not occur in small hearts because fibrillation requires the atria to have developed a critical surface area for it to occur. It is said that if you are a mouse, you can never get atrial fibrillation, whereas blue whales live in eternal atrial fibrillation, as their atria are too large to conduct synchronously. Therefore, digoxin is not contraindicated in the fetus or neonate with Wolf – Parkinson – White syndrome because the rhythm will not be atrial fibrillation, unlike older children and adults with bigger hearts. We have never seen a case of atrial fibrillation in this age group, and digoxin remains the first effective treatment of choice. Hopefully, the use of potent drugs with less familiar side effects such as flecainide can be reserved in the young for resistant cases. Most importantly, we hope that obstetricians will not feel the need to refer the mothers of these babies for magnetocardiography, elegant and fascinating as the procedure may be. Reference 1. Kaehler C, et al. Prenatal diagnosis of a Wolf – Parkinson – White syndrome by fetal magnetocardiography. Br J Obstet Gynaecol 2002;108:335 – 336. Alexander McKenzie Pirie a & John Wright b a City Hospital, Birmingham, UK b Birmingham Children’s Hospital, Birmingham, UK PII:S1470-0328(03)02836-2 The creation of twin centile curves for size Sir, In the introduction to this paper, the authors state, ‘the reason singleton centile charts are used for twin pregnancy is because twin centile charts for size do not as yet exist’. I would suggest the reason is more profound than this (i.e. that twinning in the human is a biological abnormality rather than a biological variation). This is illustrated by its relative rarity and an extraordinarily high fetal wastage. Intrauterine growth retardation is typically seen after 32 weeks of gestation, a phenomenon confirmed by this paper. The infants exposed to this growth retarding process demonstrate low to normal birth lengths and head circumferences with reduced body weight 2 . These are all the features of asymmetric D RCOG 2003 BJOG: an International Journal of Obstetrics and Gynaecology www.bjog-elsevier.com BJOG: an International Journal of Obstetrics and Gynaecology July 2003, Vol. 110, pp. 710–714

Algorithms for combining menstrual estimates of gestational age

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Page 1: Algorithms for combining menstrual estimates of gestational age

CORRESPONDENCE

Algorithms for combining menstrual estimates ofgestational age

Sir,Blondel et al.1 have demonstrated the apparent effect on rates

of preterm and postterm birth rates when changing from menstrualto early ultrasound dating. Although confidence intervals for thedifferences in the results of the two methods is not provided in thispaper, the large database of over 44,000 is likely to make theconfidence intervals very narrow.

The authors state that the ultrasound dating was based on thefetal biparietal diameter usually carried out between 16 and 18weeks but do not tell us which formula was used. No doubt thiscould have varied between the two centres and over the time of thestudy. Two commonly used published formulae (Hadlock et al.2

and Altman and Chitty3) differ by 3.5 days for a biparietaldiameter measurement of 41 mm (approximately equivalent to18 weeks of gestation).

We have carried out an audit of ultrasound dating4 in asingleton pregnancy population restricted to those pregnancieswith spontaneous onset of labour and delivery of a live healthyinfant. In this audit, dating was also based on biparietal diameter,usually between 18 and 20 weeks. The audit has shown that thepreterm rate can vary between 1.8%1 and 3.7%3 according to thetwo formulae referred above. This represents a 51% difference.

This is another factor that needs to be taken into account wheninterpreting epidemiological reports on the changes in preterm andpostterm births and we recommend that authors publish whichdating formula has been used.

References

1. Blondel B, Morin I, Platt RW, Kramer MS, Usher R, Breart G.

Algorithms for combining menstrual estimates of gestational age:

consequences for rates of preterm and postterm birth. Br J Obstet

Gynaecol 2002;109:718– 720.

2. Hadlock FP, Deter RL, Harrist RB, Park SK. Fetal biparietal diameter:

a critical re-evaluation of the relationship to menstrual age by means of

real time ultrasound. J Ultrasound Med 1982;1:97– 104.

3. Altman DG, Chitty LS. New charts for ultrasound dating of pregnancy.

Ultrasound Obstet Gynecol 1997;10:174– 191.

4. Hutchon DJR. Publishing raw data and real time statistical analysis

on e-journals, http://bmj.com/cgi/content/full/322/7285/530. BMJ 2001;

322:530.

David J. R. HutchonDarlington Memorial Hospital, Darlington, UK

PII: S 1 47 0 - 0 3 2 8 ( 0 3 ) 0 2 8 33 - 7

Prenatal diagnosis of the Wolf–Parkinson – Whitesyndrome by fetal magnetocardiography

Sir,Khler et al.1 are to be congratulated on their elegant use of fetal

magnetocardiography to detect fetal Wolf– Parkinson – White

syndrome. They tell us that this is clinically useful becausedigoxin can prove fatal in supraventricular tachycardias as a resultof Wolf–Parkinson–White syndrome, and they therefore pre-scribed flecainide.

Hopefully, we are not being too pedantic to suggest that thishas no relevance whatsoever in the fetus, neonate or small childup to the age of around one year. Digoxin is certainly contra-indicated in adults with Wolf–Parkinson–White syndrome wherethe supraventricular tachycardia may be atrial fibrillation. This isbecause digoxin paradoxically accelerates conduction in theaccessory bundle of Kent, and can cause a fatal circus rhythm.However, atrial fibrillation does not occur in small heartsbecause fibrillation requires the atria to have developed a criticalsurface area for it to occur. It is said that if you are a mouse, youcan never get atrial fibrillation, whereas blue whales live ineternal atrial fibrillation, as their atria are too large to conductsynchronously.

Therefore, digoxin is not contraindicated in the fetus orneonate with Wolf– Parkinson –White syndrome because therhythm will not be atrial fibrillation, unlike older children andadults with bigger hearts. We have never seen a case of atrialfibrillation in this age group, and digoxin remains the firsteffective treatment of choice. Hopefully, the use of potent drugswith less familiar side effects such as flecainide can be reserved inthe young for resistant cases. Most importantly, we hope thatobstetricians will not feel the need to refer the mothers of thesebabies for magnetocardiography, elegant and fascinating as theprocedure may be.

Reference

1. Kaehler C, et al. Prenatal diagnosis of a Wolf–Parkinson–White

syndrome by fetal magnetocardiography. Br J Obstet Gynaecol

2002;108:335– 336.

Alexander McKenzie Piriea & John Wrightb

aCity Hospital, Birmingham, UKbBirmingham Children’s Hospital, Birmingham, UK

PII: S1 4 7 0 - 0 3 2 8 ( 03 ) 0 2 8 3 6 - 2

The creation of twin centile curves for size

Sir,In the introduction to this paper, the authors state, ‘the reason

singleton centile charts are used for twin pregnancy is becausetwin centile charts for size do not as yet exist’. I would suggest thereason is more profound than this (i.e. that twinning in the humanis a biological abnormality rather than a biological variation). Thisis illustrated by its relative rarity and an extraordinarily high fetalwastage.

Intrauterine growth retardation is typically seen after 32weeks of gestation, a phenomenon confirmed by this paper.The infants exposed to this growth retarding process demonstratelow to normal birth lengths and head circumferences withreduced body weight2. These are all the features of asymmetric

D RCOG 2003 BJOG: an International Journal of Obstetrics and Gynaecology

www.bjog-elsevier.com

BJOG: an International Journal of Obstetrics and GynaecologyJuly 2003, Vol. 110, pp. 710–714