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Dating and growth in the first trimester

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Measurement of embryonic or foetal size using the greatest length of the embryo or foetal crown rump length can be used to accurately determine the gestational age of a normal first trimester pregnancy to within three to five days. Transvaginal ultrasound scan can be used to measure the size of an embryo and gestation sac earlier than transabdominal ultrasound. The original Robinson curve used for dating pregnancies is still valid in most cases. Ultrasound dating in the first trimester is now recommended for all women with spontaneous pregnancies, even those with certain menstrual dates.

First trimester growth in normal pregnancy is not uniform and is influenced by both maternal and foetal factors. Early foetal growth restriction is demonstrated in many pregnancies that subsequently end in first trimester miscarriage and is also demonstrated in fetuses with triploidy, trisomy 18 and possibly trisomy 13. Pregnancies which are small at the 11–14 week ultrasound scan appear to be at risk of later intrauterine growth restriction, preeclampsia and preterm delivery. Cross-sectional and serial measurement of foetal growth in the first trimester may be helpful in predicting both miscarriage and adverse late pregnancy outcomes.

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

References (66)

  • P. Kuhn et al.

    Crown-rump length in chromosomally abnormal fetuses at 10 to 13 weeks' gestation

    Am J Obstet Gynecol

    (1995)
  • F.E. Okonofua et al.

    Accuracy of prediction of gestational age by ultrasound measurement of biparietal diameter in Nigerian women

    Int J Gynaecol Obstet

    (1989 Mar)
  • H.P. Robinson

    Sonar measurement of fetal crown-rump length as means of assessing maturity in first trimester of pregnancy

    Br Med J

    (1973 Oct 6)
  • M. Nichols et al.

    Comparing bimanual pelvic examination to ultrasound measurement for assessment of gestational age in the first trimester of pregnancy

    J Reprod Med

    (2002 Oct)
  • J. Gardosi

    Dating of pregnancy: time to forget the last menstrual period

    Ultrasound Obstet Gynecol

    (1997 Jun)
  • R.L. Dutta et al.

    Patient acceptance of transvaginal sonography in the early pregnancy unit setting

    Ultrasound Obstet Gynecol

    (2003 Nov)
  • E. Jauniaux et al.

    Development of the secondary human yolk sac: correlation of sonographic and anatomical features

    Hum Reprod

    (1991 Sep)
  • S. Britten et al.

    Very early (24–56 days from last menstrual period) embryonic heart rate in normal pregnancies

    Hum Reprod

    (1994 Dec)
  • H.G. Blaas et al.

    Early development of the forebrain and midbrain: a longitudinal ultrasound study from 7 to 12 weeks of gestation

    Ultrasound Obstet Gynecol

    (1994 May 1)
  • H.G. Blaas et al.

    Early development of the hindbrain: a longitudinal ultrasound study from 7 to 12 weeks of gestation

    Ultrasound Obstet Gynecol

    (1995 Mar)
  • E. Jauniaux et al.

    The role of ultrasound imaging in diagnosing and investigating early pregnancy failure

    Ultrasound Obstet Gynecol

    (2005 Jun)
  • R.L. Deter et al.

    Individual growth patterns in the first trimester: evidence for difference in embryonic and fetal growth rates

    Ultrasound Obstet Gynecol

    (1999 Feb)
  • H.P. Robinson et al.

    A critical evaluation of sonar “crown-rump length” measurements

    Br J Obstet Gynaecol

    (1975 Sep)
  • J.E. Drumm et al.

    The ultrasonic measurement of fetal crown-rump length as a method of assessing gestational age

    Br J Obstet Gynaecol

    (1976 Jun)
  • J.E. Drumm

    The prediction of delivery date by ultrasonic measurement of fetal crown-rump length

    Br J Obstet Gynaecol

    (1977 Jan)
  • A. Selbing

    Gestational age and ultrasonic measurement of gestational sac, crown-rump length and biparietal diameter during first 15 weeks of pregnancy

    Acta Obstet Gynecol Scand

    (1982)
  • J.F. Pedersen

    Fetal crown-rump length measurement by ultrasound in normal pregnancy

    Br J Obstet Gynaecol

    (1982 Nov)
  • A.H. Adam et al.

    A comparison of crown-rump length measurements using a real-time scanner in an antenatal clinic and a conventional B-scanner

    Br J Obstet Gynaecol

    (1979 Jul)
  • L. Bovicelli et al.

    Estimation of gestational age during the first trimester by real-time measurement of fetal crown-rump length and biparietal diameter

    J Clin Ultrasound

    (1981 Feb)
  • D.M. Lasser et al.

    First-trimester fetal biometry using transvaginal sonography

    Ultrasound Obstet Gynecol

    (1993 Mar 1)
  • S.N. MacGregor et al.

    Underestimation of gestational age by conventional crown-rump length dating curves

    Obstet Gynecol

    (1987 Sep)
  • F.P. Hadlock et al.

    Fetal crown-rump length: reevaluation of relation to menstrual age (5–18 weeks) with high-resolution real-time US

    Radiology

    (1992 Feb)
  • A. Kustermann et al.

    Transvaginal sonography for fetal measurement in early pregnancy

    Br J Obstet Gynaecol

    (1992 Jan)
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