First trimester fetal growth restriction and cardiovascular risk factors in school age children: population based cohort studyBMJ 2014; 348 doi: https://doi.org/10.1136/bmj.g14 (Published 23 January 2014) Cite this as: BMJ 2014;348:g14
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Re: First trimester fetal growth restriction and cardiovascular risk factors in school age children: population based cohort study
Jaddoe and colleagues report a relationship between first trimester fetal growth, as measured by ultrasound crown rump length (CRL), and cardiovascular risk in childhood (1). But CRL is a measurement of fetal size, not of growth. The authors infer growth from observed first trimester fetal size in relation to the first day of the last menstrual period (LMP).
Just over a decade ago Allen Wilcox showed that irrespective of known LMP, embryonic implantation may vary by several days and the later implantation occurs, the higher is the risk of first trimester miscarriage (2). There has recently been significant interest in uterine selection of embryos at implantation and the impact this may have on pregnancy outcome (3).
Estimating day of ovulation, fertilization and implantation (hence gestation) from the first day of the last menstrual period is imprecise even in women with regular menstrual cycles: errors of many days may occur. Using sensitive urinary ovulation and pregnancy testing kits with repeat ultrasound CRL measurements, we have shown that the major determinant of first trimester fetal size is not growth (which remains remarkably constant), it is implantation day (4). The important findings of Jaddoe et al’s study cannot be assumed to be related to fetal growth in the absence of a measure of fetal growth.
1. Jaddoe V, de Jonge L, Hofman A, Franco O, Steegers E, Gaillard R. First trimester fetal growth restriction and cardiovascular risk factors in school age children: population based cohort study. BMJ 2014;348:g14
2. Wilcox AJ, Baird DD, Weinberg CR. Time of implantation of the conceptus and loss of pregnancy. N Engl J Med 1999 Jun 10;340(23):1796-9.
3. Brosens JJ, Salker MS, Teklenburg G, Nautiyal J, Salter S et al. Uterine selection of human embryos at implantation. Sci Rep. 2014 Feb 6;4:3894. doi: 10.1038/ srep03894
4. Mahendru AA, Daemen A, Everett TR, Wilkinson IB, McEniery CM, Abdallah Y,Timmerman D, Bourne T, Lees CC. Impact of ovulation and implantation timing on first-trimester crown-rump length and gestational age. Ultrasound Obstet Gynecol.2012 Dec;40(6):630-5.
Competing interests: CL and TB are supported by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Imperial College Healthcare NHS Trust and Imperial College London. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.