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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
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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
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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