16 18 20 22 24 26 28 30 32 34 36 38 40 Gestational age (weeks)
Chapter 15 Prenatal Ultrasound Measurements Prenatal Measurements
In general, ultrasound measurements use the longest dimension of a particular structure, whether it is a bone such as the femur or an organ such as the kidney. Ratios help to determine whether there is disproportionate growth, undergrowth, or overgrowth of a structure. When assessing brain growth, ventricular size must be evaluated (Fig. 15.21), and when assessing renal size and function, the amount of amniotic fluid must be evaluated (Fig. 15.23).
Normal Fetal Activity During Gestation
Patterns of fetal activity indicate fetal well-being and help to determine maturation.
Fig. 15.24 Fetal movement during gestation,
Type of movement Normal activity
Embryo movement Limb movement
Swallowing Bladder emptying
From 7 wk
From 10 wk, increases up to 20 wk Decreases from 20-40 wk 12-24 wk 28-40 wk
30 wk onwards
From 12 wk
From 12 wk
From 12 wk
Mean movement 120/hr Mean movement 60-80/hr Seen 5-17% of time Present 30% of time; episodic and irregular; may go up to 2 hr without breathing movements 20-60 breaths/min Coordinated and synchronized breathing movements Abrupt thoracic and abdominal movement Hands are held around the face; regular thumb sucking in third trimester Amniotic fluid increases if no swallowing occurs Fills over 45 min period, then empties
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