INTERGROWTH-21
st is a multicenter, multiethnic, population-based project, conducted between 2009 and 2014 in eight urban areas in eight different countries: the cities of Pelotas, Brazil; Turin, Italy; Muscat, Oman; Oxford, UK; Seattle, USA; Shunyi County, Beijing, China; the central area of the city of Nagpur (Central Nagpur), Maharashtra, India; and the Parklands suburb of Nairobi, Kenya13 (
link). Its primary aim was to study growth, health, nutrition and neurodevelopment of fetuses from < 14 + 0 weeks' gestation to 2 years of age, using the same conceptual framework as the World Health Organization (WHO) Multicentre Growth Reference Study12 (
link), in order to produce prescriptive growth standards to complement the existing WHO Child Growth Standards14 .
These urban areas had to be located at low altitude (≤ 1600 m) and women receiving antenatal care had to plan to deliver in these institutions or in a similar hospital located in the same geographical area, and there had to be an absence or low levels of major, known, non-microbiological contamination such as pollution, domestic smoke, radiation or any other toxic substances, evaluated during the study period at the cluster level using a data collection form specifically developed for the project15 (
link). In the eight urban areas, we selected all institutions providing pregnancy and intrapartum care, in which > 80% of deliveries occurred.
To generate the CRL data for our stated aims, women with a singleton pregnancy that was conceived naturally were asked to participate in the Fetal Growth Longitudinal Study (FGLS), one of the three main components of the INTERGROWTH-21
st Project, whose study methods have been described in detail elsewhere13 (
link). Briefly, we recruited women from the selected populations with no clinically relevant obstetric or gynecological history, who met the entry criteria of optimal health, nutrition, education and socioeconomic status to create a group of affluent, clinically healthy women who were at low risk of intrauterine growth restriction and preterm birth. Recruitment occurred prospectively and consecutively at 9 + 0 to 13 + 6 weeks' gestation as estimated by LMP provided that: (1) the date was certain; (2) the agreement between LMP and CRL dating was ≤ 7 days; (3) the women had a regular 24–32-day menstrual cycle; and (4) they had not been using hormonal contraception or breastfeeding in the preceding 2 months. The women, who were all well-educated and living in urban areas, reported the date and certainty of their LMP at their first antenatal clinic visit in response to specific questions.
A single type of ultrasound machine (Philips HD-9; Philips Ultrasound, Bothell, WA, USA) with an abdominal probe was the machine of choice to measure CRL. However, as the first contact with the study often occurred at several different clinics in the geographical area, it was considered acceptable to use other, locally available, machines for the CRL measurement at the first antenatal visit only, provided that they were evaluated and approved by the study team. All 39 ultrasonographers at the eight study sites underwent rigorous training and standardization specifically for CRL measurement16 (
link). In accordance with the study's quality-control protocol, they also submitted images of the CRL measurements, which were reviewed blindly by our collaborators at the Société Française pour l'Amélioration des Pratiques Echographiques. The ultrasonographers were only certified to measure CRL in the study if they demonstrated adequate knowledge of the study protocol and the quality of the images submitted for review was satisfactory17 (
link).
CRL was measured once using strict techniques and imaging criteria18 (
link). A discrepancy between GA based on LMP and that derived from CRL of more than 7 days was a reason to exclude the woman from the study. All women were then followed to delivery with standardized antenatal care evaluation and regular ultrasound scans every 5 ± 1 weeks.
The INTERGROWTH-21
st Project was approved by the Oxfordshire Research Ethics Committee ‘C’ (ref: 08/H0606/139) and the research ethics committees of the individual participating institutions, as well as the corresponding regional health authorities in which the project was implemented.
Papageorghiou A.T., Kennedy S.H., Salomon L.J., Ohuma E.O., Cheikh Ismail L., Barros F.C., Lambert A., Carvalho M., Jaffer Y.A., Bertino E., Gravett M.G., Altman D.G., Purwar M., Noble J.A., Pang R., Victora C.G., Bhutta Z.A, & Villar J. (2014). International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown–rump length in the first trimester of pregnancy. Ultrasound in Obstetrics & Gynecology, 44(6), 641-648.