Which measurement is most accurate for estimating gestational age in the third trimester?

Crown–Rump Length

Crown–rump length in large animals, such as dogs and minipigs, can supplement body weight as a measure of normal growth. In rodents, crown–rump length has been highly correlated with body weight. The crown–rump length can be measured at various ages during the postnatal period, and normal curves can be established. When body weight measurements are automated, they are more consistent and accurate than crown–rump length, which is measured with calipers and is subject to user bias. Compounds that cause edema may lead to changes in crown–rump length that will not always correlate with body weight, but this level of edema is usually evident to the naked eye.

CROWN–RUMP LENGTH

The CRL is the longest length of the embryo or fetus measurable excluding the limbs and yolk sac.14 The embryo becomes a fetus after 10 gestational weeks (71 completed days based on the LMP). The accuracy of the CRL in dating the pregnancy depends on good correlation between this measurement and fetal age in a period when growth is rapid and minimally influenced by fetal pathology. The CRL is predictive of fetal age with an error of 3 days (90% confidence limits) from 7 to 10 weeks and of 5 days from 10 to 14 weeks' gestation. The CRL grows approximately 10 mm per week from weeks 8 to 12 and a simple rule to obtain GA is the following: GA (week) = CRL (cm) + 6.5.14,22

Optional Assessments

Crown-rump lengths, anogenital distance, organ weights, double-staining of the fetuses with alizarin red S and alcian blue to facilitate a thorough examination of the skeleton and the cartilage, and maternal and/or fetal toxicokinetics (eg, blood samples, amniotic fluid, placentas) are all fetal assessments that can be performed if deemed appropriate based on the nature of the test article under investigation.

It should be recognized that a complete assessment of developmental toxicity includes an evaluation of functional deficits that can be more appropriately performed in the F1-generation offspring naturally delivered from F0-generation animals exposed to a test agent during pregnancy. This study design is the ICH 4.1.2, which is fully described elsewhere in this chapter.

Modifications to the embryo-fetal developmental study design are based on the test agent and can include, but are not limited to, initiation of dose administration prior to implantation, continuation of dosing after the closure of the hard palate, or “pulse” dosing through the period of major organogenesis. In addition, in cases where the test agent is not expected to pose any reproductive or developmental issues then the embryo-fetal developmental study can be incorporated into the fertility study design to address both male and female fertility, reproductive performance, and developmental toxicity, as previously described.

PREGNANCY DATING

The crown–rump length (CRL) of the fetuses is the most important ultrasound parameter for dating of the pregnancy and, if necessary, to correct the gestational age in cases with a non-reliable menstrual history. The onset of early growth retardation in one of the fetuses may indicate a higher risk for chromosomal abnormalities.

Normally, the CRLs correlate between co-twins, although some degree of variability has been observed in multifetal pregnancies. 19,34 Measurement of the CRL can easily be done at the time of the first-trimester scan (11–14 weeks of gestation). Later in pregnancy, correction of gestational age should be avoided, since growth curves in multiple pregnancies differ from those in singleton pregnancies beyond the second trimester.

Crown–rump length (CRL)

Between 6 and 12 weeks gestational age (GA), the measurement of the crown–rump length (CRL) of the embryo is most accurate for dating. The now classical study by Robinson and Fleming31 on CRL is still the main reference for the assessment of gestational age in early pregnancy. There are no significant differences with measurements made by the transabdominal route or the transvaginal route. Because transvaginal scan provides superior resolution and more accurate identification of the embryonic structures than abdominal ultrasound, new charts have been developed for the period of gestation before 7 weeks. The original data of Robinson has been extrapolated from embryos of 7 mm in size down to embryos of 2 mm with preservation of accuracy and reliability.31

The accuracy of CRL measurements in determining gestational age is within 3–5 days. There is an optimum window for CRL measurement which is between 7 and 9 weeks using transvaginal scan. The confidence interval at 8 weeks is ±0.64 weeks and at 12 weeks is ±0.96 weeks.

Assessment of gestational age

Measurement of the mean gestational sac diameter is the first parameter that can be used to determine gestational age.

The rate of growth of the normal gestation sac is 1.1 mm/day.

Measurement of the crown–rump length (CRL) between 6 and 12 weeks is the most accurate dating parameter.

CRL measurements of gestational age are accurate to within 3–5 days.

MEASUREMENTS OF THE EMBRYO/EARLY FETUS

The crown–rump length (CRL) is measured as the greatest length in a straight line from the cranial to the caudal end of the body in the straightest possible position of the embryo/fetus (Fig. 4.7). The CRL diagram presented by Robinson in 1975 is still widely used for the evaluation and dating of the early pregnancy.11

The width of the head, also designated as the biparietal diameter (BPD), is measured in the horizontal section perpendicular to the body axis. Due to the development of the brain, the largest width alters its position in relation to cerebral landmarks during the embryonic and early fetal period (Fig. 4.8). At 7 weeks, BPD is measured at the height of the rhombencephalon. In the early fetal period the future cranium becomes more distinguished such that the BPD can be obtained by placing the calipers at the outer border of the not yet ossified cranium in a horizontal section at the level of the thalamus. The anteroposterior diameter of the embryonic head, designated as the occipitofrontal diameter (OFD), is measured in the same section perpendicular to the BPD. The embryonic head circumference (HC) measurement is usually calculated from the BPD and the OFD, using the formula for an ellipse.

Measurements of the embryonic trunk (abdominal circumference, AC) have been introduced as a possible parameter for the estimation of embryonic age during first-trimester biometry.12 It is advantageous to use comparable parameters when describing embryonic and fetal biometry. Instead of measuring the mean diameter of the abdomen and multiplying it with the constant 2π to obtain the AC, one may use a simpler parameter such as mean abdominal diameter (MAD)9 alone. This parameter is derived from two perpendicular measurements taken in the horizontal plane through the upper embryonic abdomen below the heart and above the umbilicus/midgut herniation (Fig. 4.9). Longitudinal examinations of the BPD and MAD in 29 normal pregnancies showed that the growth of the healthy embryo is constant (Figs. 4.7, 4.10).9

In 1973, Robinson showed that the heart rate reached a maximum at 9 weeks in the first trimester.13 The heart can easily be recognized by real-time ultrasound as a relatively large beating structure below the embryonic head. The heart rate should be analysed electronically using the M-mode facility (Fig. 4.11). ‘Manual’ counting results in lower maximum heart rates, which again may result in incorrect counselling and poorer management of the patient. In normal pregnancies, the heart rate develops in a specific pattern, increasing from approximately 100 bpm at the end of 5 weeks to a peak mean of 175 bpm at 9 weeks, and slowly decreasing to 150 bpm in the second trimester (Fig. 4.12).

The physiological midgut herniation is recorded by measuring the length of the protruded bowel into the cordal coelom. It is usually detectable at 8 weeks, has its maximal extension at the beginning of week 10, and can be seen until the end of week 11.8,14,15

Significant ossification of the long bones is not seen before 10 LMP weeks and later.16 This was confirmed in a study on the development of the skeleton comparing longitudinal ultrasound imaging from living embryos/fetuses with radiographs obtained from aborted silver nitrate-impregnated embryos and fetuses.10 At 10.5 weeks the ossified part of the femur was just measurable to 2.1 mm by ultrasound. In a 10-week-old silver nitrate-impregnated embryo, the femur length was even shorter. Therefore, measurement of limbs does not have clinical significance in the first trimester.

INTRODUCTION

The birth weights, crown-rump lengths, and head and chest circumferences of living newly-born infants of a given gestational age may vary widely, so these data alone may not be sufficient to assess the maturity of an infant at birth. Additional physical information, such as the amount of ear cartilage and breast tissue or the number of sole creases, is helpful in estimating more accurately the baby's age, as are the neurodevelopmental responses, such as postural or reflex activity and the EEG patterns. These supplementary methods of estimating maturity have been standardized, allowing more accuracy in determining the gestational age if it is suspected to differ from the age calculated according to the mother's last menstrual period.

Among nonsurviving newborns simple measurements, such as cerebral and visceral weights, may be of limited value in estimating the maturity of the infant because of the variation among these data at any given age (Chap. 6). Additional criteria of growth of the brain, such as the number and complexity of the gyri, can assist the assessment of the maturation of a specific neonate.

In the past, the regional development of fetal brains has been described with reference to brain weight (Rorke and Riggs, 1969), broad gestational periods (Retzius, 1896), restricted periods of embryogenesis (Hochstetter, 1929), or maturation of other fetal organs (Dorovini-Zis and Dolman, 1977). We have studied the sequence of fissural, sulcal, and gyral development of the brains of infants in the NINCDS Collaborative Perinatal Project (NCPP) who ranged in gestational age (calculated according to the mother's last menstrual period) from 10 to 44 weeks (Chi, Dooling, and Gilles, 1977; Chi and Dooling, 1976). Although Dorovini-Zis and Dolman (1977) found the base of the brain not useful in estimating gestational ages, we used all surfaces. Our findings provide data that can be used as standards. In this way, inspection of a specimen will disclose if it deviates from the norms we have established in terms of being retarded or accelerated in its differentation or having an unusual external appearance.

RESULTS

Steroidogenesis by the early fetal testes

The data presented are based on fetal CRL (cm) and known gestational ages. The CRL as a function of fetal age is shown in Fig. 1. The undifferentiated gonads at 30–40 days, were shown to produce progesterone but not testosterone or estrogen, regardless of the sex of the fetus, and did not respond to LH.

Fig. 1. Plot of fetus crown-rump length in cm as a function of fetal age

(from ref. [19]).

At 43 days of gestation the individual fetus began to produce testosterone in ng amounts and this was observed throughout the first trimester (Fig. 2). Between 45 and 110 days the amount of testosterone produced per mg declined from 1703 ± 391 pg/mg gonad/24 h to 37 ±16 pg/mg gonad/24. This may have been due to the rapid growth of the fetal testes during this period (from 2 to 75 mg). Fetal testes from fetuses of 3.3–20 cm CRL were responsive to LH (Fig. 3). However, while testosterone was elevated significantly, progesterone production was not affected during the entire period studied.

Fig. 2. Semilogarithmic plot of cultured gonad testosterone accumulation after 24 h incubation (ng/gonad per 24 h) as a function of fetal crown-rump length

(from ref. [19]).

Fig. 3. Testosterone secretion (m ± SE) by the early bovine fetal cultured testes (n = 5–13) after 24 h incubation with or without added LH 1 μg/ml to the medium (TCM-199) in relation to fetal size. LH enhanced testosterone secretion significantly (P < 0.05) by all testes from 3.3 to 12.4 cm crown-rump length

(from ref. [19]).

Steroidogenesis in the fetal ovary

Similar to the testes, the fetal ovary also showed a marked initial rise in steroidogenesis at the time of gonadal differentiation. However, the principal product of the ovary was 17β-estradiol which was produced in ng amounts (Fig. 4). The initial production of 2674 ± 512pg/mg gonad/24 h incubation at 3.3–4.5 cm dropped rapidly to 286±176 pg/mg gonad/24 h at 7.8–8.5 cm CRL. This could not be attributed to weight changes as the ovary grew only from about 2 to 7 mg during this time. Furthermore, 17β-estradiol production was undetectable in all older fetuses, while testosterone was readily detectable in testes of the same size. The mean values of 17β-estradiol secreted by the cultured ovaries from fetuses of 3.4–8.1 cm CRL (42–80 days) was significantly enhanced by the presence of bovine LH (Fig. 5 and Table 2).

Fig. 4. Semilogarithmic plot of cultured gonad 17β-estradiol accumulation after 24 h incubation (ng/gonad per 24 h) as a function of fetal crown-rump length

(from ref. [19]).

Fig. 5. 17β-Estradiol secretion (m ± SE) by the early bovine fetal cultured ovaries (n = 4–8) after 24 h incubation with or without added LH (1 μg/ml) to the medium (TCM-199) in relation to fetal size. LH enhanced significantly 17β-estradiol secretion (P < 0.05) only by ovaries from 3.3 to 4.1 cm crown-rump length

(from ref. [19]).

Table 2. Effect of LH or testosterone on 17β-estradiol accumulation in incubated fetal ovaries. Values are expressed as ng/ovary per 24 h incubation (mean ± SEM) from ref. [20])

		Treated
Crown-rump length (cm)	Control*	LH-	Testosterone
5–8	1.5 ± 0.4 (n = 26)	4.2 ± 0.6 (n = 13)	6.5 ± 0.7 (n = 13)†
10–20	ND(n = 49)	ND(n = 6)	2.2 ± 0.4 (n = 24)†

*One of each pair of ovaries served as a control while the other was treated.†P < 0.001.

Prostaglandin F2α was secreted by the cultured testes and ovaries from all the fetuses studied over the entire range of fetal size. Similarly, prostaglandin E2 was secreted by both cultured testes and ovaries. However, in contrast to the testes, prostaglandin E2 secretion was significantly reduced in the “older” ovaries (CRL > 8).

CHAPTER 9 FETAL BIOMETRY, ESTIMATION OF GESTATIONAL AGE, ASSESSMENT OF FETAL GROWTH

1

What is the aim of CRL measurement?

to determine fetal weight

to determine true gestational age

both

other aims

The accuracy of pregnancy dating:

increases with gestational age

decreases with gestational age

is independent from gestational age

The measurement of biparietal diameter:

is used for dating pregnancy in the second trimester

is very accurate for dating pregnancy in the third trimester

is taken at the same level as the measurement of the cerebellar diameter

The abdominal circumference:

is used for fetal dating in the third trimester

is used for the evaluation of fetal growth disturbances in the second trimester

is the main factor in fetal weight determination in most of the mathematical equations

For a confident diagnosis of IUGR:

the calculated fetal length should be below normal values

the calculated fetal weight should be below normal values

both AC and fetal weight estimation, made at least 2–3 weeks apart, should be below normal values

Questions

1.

Fetal growth:

Crown–rump length is considered to be an accurate ultrasound measurement of fetal growth up until 14 weeks

The biparietal diameter is considered to be an accurate ultrasound measurement of fetal growth from 10 weeks

The head/abdominal circumference ratio decreases with advancing pregnancy

Fetal weight increases with increasing parity

In late pregnancy occurs at the same rate as placental growth

Fetal circulation:

Circulation of blood is present by 21 days

The heart originates in splanchnic mesenchyme

The fetus responds to hypoxia by increasing the circulation to adrenal glands

In response to hypoxia, the fetus can increase the placental circulation by 50%

In utero only 1% of the cardiac output is directed to the lungs

Renal function:

Renal agenesis is associated with oligohydramnios

The kidney at term is the principal source of amniotic fluid

In late pregnancy the amniotic fluid osmolarity rises

The concentration of creatinine in amniotic fluid is less than in the fetal plasma

The rate of fetal urine production can be measured by ultrasound

Nervous system:

Nerve cell processes in the cerebrum appear at 8–10 weeks

Nerves appear in the fetus between 4 and 5 weeks

Limb muscle fibres can contract by 8 weeks

Baroreceptor stimulation of the aortic branches of the Xth cranial nerve results in a bradycardia

Chemoreceptors in the carotid body respond in utero to alterations in pH

Alimentary tract:

Swallowing commences at 20 weeks

Meconium contains vernix

Peristalsis is inhibited by hypoxia

The stomach bubble on ultrasound is only visible after 20 weeks

Digestive enzymes are found by 12 weeks

Respiratory system:

Breathing movements are present from 20 weeks

Alveolar development is complete by 24 weeks

Type I pneumocytes produce surfactant

Intrathoracic pressures of up to –10 cm H2O may be required for the baby's first breath

As labour approaches, fetal breathing decreases

Placental transfer:

The placenta is a complete barrier to substances with molecular weights greater than 1500

3–4 L of fluid are exchanged between the mother and fetus per hour

The placenta converts phospholipids to simpler forms

The concentration of amino acids is less in the fetal blood than in the maternal blood

Oxygen delivery occurs by diffusion rate from mother to fetus

Oxygen and carbon dioxide transfer:

The oxygen dissociation curve for fetal blood is shifted to the right

Fetal blood has a larger carrying capacity for oxygen than maternal blood

In low oxygen tensions fetal Hb has lesser affinity for oxygen than maternal Hb

Carbon dioxide is mostly carried in the blood as carbonic acid or bicarbonate

The Haldane effect facilitates release of carbon dioxide from fetal haemoglobin

Oxygen:

The fetus at 16 weeks of gestation requires 25 mL/min of oxygen

Each gram of Hb combines with 1.34 mL of oxygen

HbF binds 2, 3-DPG less effectively

At P50, the Hb dissociation curve of the fetus is less steep than that of the mother

In fetal hypoxia, both metabolic and respiratory acidosis occur

Oxygen and carbon dioxide:

The fetus requires oxygen at higher tensions than that of the mother

A fall in plasma pH decreases the affinity of red blood cells for oxygen

Excess fetal lactic acid is metabolized in the placenta

Excess carbon dioxide dilates placental vessels

The fetus may become polycythaemic in response to low oxygen tensions

Placental structure:

At term new placental villi continue to be formed

There is a positive correlation between placental weight and fetal weight

Microvilli are present on the vasculo-syncytial membranes

The main factor governing the rate of placental blood flow is the vascular resistance of the spiral arteries

At term the total blood flow to the placenta is about 500 mL

The following organisms cross the placenta:

Variola vaccinia

Coxsackie virus

Listeria

Neisseria meningitidis

Toxoplasma

The average weight gain at term of:

the uterus is 400 g

the breasts is 400 g

the blood is 1200 g

the placenta is 600 g

the liquor is 800 g

The placenta produces the following substances:

oestrone

5α-reductase

oxytocinase

histaminase

progesterone