Alternate methods of assessing GA are therefore necessary in such settings. Dating the last menstrual period LMP and measuring the symphysis-fundal height SFH are alternate means of assessing GA, but each has limitations. Dating the LMP may be difficult because of poor recollection, irregular menstrual cycles of varying duration, lactational amenorrhea, bleeding in early pregnancy, or hormonal contraceptive use prior to conception [ 1 , 2 , 3 , 4 , 7 , 8 ].
The SFH may be used after 12 weeks of gestation but its accuracy may be diminished by a multiple pregnancy, maternal size, intrauterine growth restriction, fetal position, and other maternal or fetal characteristics [ 9 , 10 , 11 , 12 ]. Moreover, due to the observed variations in SFH across populations, local standards are required for optimal pregnancy dating using SFH [ 13 , 14 , 15 , 16 , 17 ].
Using the US scan as the gold standard, we evaluated LMP dating and SFH measurement as alternate tools for estimating GA between 20 and 26 weeks of pregnancy, and thus for predicting the date of delivery and estimating GA at delivery. The study was reviewed and approved by the Ethical and Review Committee of Aga Khan University and the Review Boards of the University of Alabama and Research Triangle Institute.
Material and methods This prospective, population-based cohort study was conducted in 4 administrative units in Hyderabad, Pakistan. The study area comprised a low- to middle-income population of about 90 residents. The pregnant women were identified by government lady health workers LHW , who are each responsible for providing basic maternal and child care to households. The LHWs visit each household under their care once per month and maintain logs of all pregnancies and birth outcomes.
We worked with 90 LHWs after training them in the research protocol, study recruitment, and confidentiality. The study nurse supervised the LHWs in their study-related field work. The LHWs explained the study to the pregnant women and screened them for eligibility. The criteria were being 16 years or older and between 20 and 26 weeks of pregnancy; having no serious medical condition; and being a resident of the area planning to be delivered in the area.
The eligible women were scheduled for a visit at the research clinic closest to their homes. During the visit GA was determined by ultrasound to confirm eligibility, and other data were collected on pretested forms by trained research staff that included 2 women physicians, 2 LHWs, and 1 midwife. The LHWs regularly visited all enrolled women until delivery. Within the 48 hours that followed delivery the study physician and a nurse visited the woman to collect information concerning the delivery and other maternal and neonatal particulars.
All women gave informed consent prior to participating in the study. Conclusions We have produced international prescriptive standards for early fetal linear size and ultrasound dating of pregnancy in the first trimester that can be used throughout the world. At a population level, GA estimation is required to determine rates of small-for-gestational-age fetuses and preterm birth accurately in order to allocate resources appropriately 4 , 5. GA has traditionally been calculated from the first day of the last menstrual period LMP.
However, in a proportion of pregnancies, depending on the locality, the LMP is unknown or the information is unreliable 6 , 7. Between 9 and 13 weeks' gestation, linear growth evaluated by CRL is rapid and the SD is rather small, which means that GA can be estimated accurately. In later pregnancy, head circumference is typically used for dating, as CRL can no longer be measured owing to curling of the growing fetus; however, variation is greater, which results in less accurate estimation of GA 9.
For this reason, first-trimester ultrasound estimation of GA is recommended in clinical practice 8. Various studies have been conducted to derive CRL reference charts for the estimation of GA, mostly in single institutions or geographical locations. A review of their methodological quality has shown several limitations including highly heterogeneous study designs and approaches to statistical analysis and reporting This could be achieved by first selecting pregnant women at low risk for fetal growth impairment, living in environments with minimal exposure to factors that have an adverse effect on growth.
From such populations, women at low risk of adverse pregnancy outcomes who deliver healthy newborns without congenital malformations would then be identified 11 — Our aim in this study was to generate CRL data according to GA using an optimal study design and prescriptive approach in order to develop international, population-based standards for early fetal linear size estimation and ultrasound dating of pregnancy in the first trimester that can be used throughout the world.
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. 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 measurement 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 satisfactory CRL was measured once using strict techniques and imaging criteria 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.
First Trimester / Dating Ultrasound
International standards for early fetal size and pregnancy dating based on ultrasound measurement of crown–rump length in the first trimester of pregnancy
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