Best Practice & Research Clinical Obstetrics & Gynaecology
2Dating and growth in the first trimester
Section snippets
Normal developments
In order to make and interpret measurements of a pregnancy in the first trimester, it is important to understand the normal development of the embryo and fetus and how they relate to what is visualized with ultrasonography.
From the time of fertilization until implantation the visible ultrasound findings suggesting a potential pregnancy are the presence of a thickened hyperechogenic homogenous endometrium (the decidual reaction) and a corpus luteum on the ovary (solid, cystic or haemorrhagic,
Measurement of the gestation sac
Measurements of the gestation sac are taken in three orthogonal planes (usually two in a sagittal plane and one in transverse, using the inner borders of the sac) from which the mean diameter or volume can be calculated.
Measurement of the yolk sac
The yolk sac is also measured in three orthogonal planes, as for the gestation sac. It is usually measured from the outer borders of the sac.
Measurement of the embryo or fetus
Initially, when the embryo is 1–2 mm in length (5–6 weeks, Carnegie stage 8–9) it is the straight line length (“greatest length”) that is
First trimester dating of pregnancy
The estimation of gestational age using ultrasound measurements of the embryo or fetus was described by Robinson in 1973. He used a transabdominal static scanner to obtain images from 80 women with certain menstrual dates from 6 to 14 weeks gestation.*1, *16 It was estimated that menstrual age could be predicted with 95% confidence intervals of ±4.7 days on a single measurement or 2.7 days if three independent measurements of the same fetus were used. It is generally considered remarkable that
Variation in first trimester growth
It is often assumed that growth is uniform in the first trimester.40, 41 However, variation in foetal growth may occur due to a host of maternal factors, such as age, smoking history, ethnicity, body mass index or vaginal bleeding. It may also be due to foetal factors and in particular chromosomal abnormality. Very early ultrasound work showed that deviation from normal growth rates was suggestive of abnormal pregnancy development.28 Recent work has focused on how “redating” at the end of the
Summary
Transvaginal ultrasound has largely superseded transabdominal assessment in the early first trimester due to the ability to obtain high resolution images of the early pregnancy and to diagnose failed pregnancies at an earlier gestation. However, the accuracy of transvaginal ultrasonography has not been shown to be greatly superior in dating pregnancies to the original transabdominal measurements of foetal size published over 30 years ago.
Dating by ultrasound is more accurate than by menstrual
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