Early Pregnancy Fetal Heart Beat – Now you see it, now you don’t

Images and text Genevieve Carbonatto

PV bleeding in early pregnancy is common. This is probably one of our most common presentations to the emergency department. Because we only have access to transabdominal ultrasound, the sensitivity of this exam in first trimester pregnancy is not as good as it is for transvaginal ultrasound. Despite this we need to exclude emergencies and therefore seeing an intrauterine gestational sac is reassuring that we do not have an ectopic pregnancy. Clearly a heterotropic pregnancy is always a possibility, but using our clinical judgement and the BHCG levels  we can usually be reassured once the intrauterine sac is visualised and there is no fluid in the pouch of Douglas.

The next step is to see whether we have a viable intrauterine pregnancy. This means that we need to have seen a heart beat.

The following 3 ultrasound cases will focus on this.

This is the ultrasound of a 7 week (by dates) pregnant lady. No previous formal ultrasounds done.

There is  an intrauterine gestational sac

By zooming in on the gestational sac, we have an intrauterine pregnancy which measures 1.08 cm

snapshot50

We then focus on the fetal heart

Do you see a fetal heart beat? At times it looks like there might be one, at times you can’t see it. It is a little like the picture at the start of this post, like a visual illusion. Keep in mind that you have seen and measured a substantial fetal pole and you might be wanting very much to reassure this lady that her pregancy is viable. There is what appears to be motion within the fetal pole, but is this motion a fetal heart beat or is it transmitted from maternal blood flow, from her heart beat and  respiration or from fluid moving within the maternal bowel?  There is in fact no heart beat and it is not possible to put an M mode through the heart to measure  it. Does this mean there really isn’t one? No. All you can say is that with transabdominal ultrasound you cannot adequately display the fetal heart beat and that there is an intrauterine pregnancy but a transvaginal ultrasound will determine fetal viability.

The following is a better view of the fetal pole and here a heart beat is not visualised.

On transvaginal scan an absent heart beat was confirmed.

Now have a look at this pregnancy. This lady is also 7 weeks pregnant by dates. She also has PV spotting.

This time the fetal heart beat is clear. A small regular flicker can be seen in the fetal pole.  Only ever use 2D guided M-mode on fetal hearts.  Please never use 2D guided Doppler of any description.

heart-beat

M mode enables the calculation of the heart beat. It is 144 bpm. The M mode cursor is placed through the heart and the regular flicker of the heart beat can clearly be seen on the M mode trace.

This is another early pregnancy scan. The fetal pole is larger, 2.25 cm and therefore easier to see.

M mode again confirms the regular fetal cardiac activity.

cardiac-actvity-2

 

Teaching point: Be careful when confirming a heart beat in an early uterine pregnancy on a transabdominal scan. If you cannot get an adequate M mode which demonstrates the heart beating then you have not identified a heart beat. Confirmation with a formal transvaginal scan should be organised as soon as possible.

 

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