OB Ultrasound Basics

Normal Maternal Anatomy-

Always need a picture of the cervix. < 3.5 cm in length is shortened. Look for early signs of opening or "funneling".
If the placenta is within 1.5 cm of the cervical os, but not covering it, it is a low lying placenta. If covering it, it is placenta previa.

Normal fetal anatomy-

Posterior fossa should be less than 1 cm.
4 chamber heart.
Pericardial effusion is normal if less than 2mm.
Ventricular atrium should be less than 10 mm.
Stomach on left and not distended.
Normal kidneys without hydronephrosis.
Bladder is nondistended.
3 vessel umbilical cord.
Cord insertion with space on both sides.
Posterior skin line or spine can only be called normal if it is not up against anything, otherwise, it is limited.
Nuchal lucency <3 mm at 11-14 weeks.
Nuchal thickness <6 mm at 15-16 weeks.
Head shape is oval. If elongated= dolichocephaly. If round= brachycephaly.

Gestational age on ultrasound should be within 2 weeks of clinically estimated age in order to be congruent.
If more than 2 weeks different, the dates are discordant which could mean intrauterine growth restriction (which requires follow up).

What you should see at certain dates-

Gestational sac at 5 weeks. It should grow at approximately 1.2 mm per day.

Yolk sac at 5-6 weeks, or, when the mean gestational sac diameter (MGSD) is 8 mm (Transvaginal) or 20 mm (Transabdominal).

Embryo/ Fetal pole at >6 weeks, or, when the mean gestational sac diameter (MGSD) is 16mm (Transvaginal) or 25 mm (Transabdominal).

Heart rate should be seen when the crown rump length is greater than 5 mm.
HR >100 at 5-6 weeks.
HR 120-180 in 2nd and 3rd trimesters.

Amniotic Fluid Index (AFI)

5-10 percentile= Intrauterine growth restriction (IUGR)
11-17 percentile= Normal
18-28 percentile, or, if largest fluid pocket is greater than 8 cm= Polyhydramnios