SASUOG

South African Society for Ultrasound
in Obstetrics and Gynaecology

TEACHINGS / LECTURES / CASES

Assessment of early pregnancy prognosis

Many pregnancies are "lost" due to a natural attrition, especially in the early first trimester. Many of these pregnancies losses, which might otherwise have gone unnoticed, or would been ascribed to a heavier than usual menstruation, are nowadays recognised thanks to sensitive biochemistry and ultrasound. Unfortunately, some patients have unrealistic expectations of a "single perfect pregnancy", and believe that once the pregnancy test has come up positive, and a gestational sac has been seen on scan, the baby is as good as delivered. On the other hand, the old adage of "wait until 12 weeks before telling anyone" can cause unnecessary anxiety in someone with a pregnancy with a good prognosis.

If you regularly (or irregularly) examine early pregnancies by ultrasound, you should know which findings are reassuring, and which not, and when to request biochemistry (ßHCG- or progesterone levels). Also remember that there are no absolutes: a pregnancy with reassuring signs still has a (small) chance of miscarrying, and a pregnancy with signs pointing to inevitable pregnancy failure, still has a (very small) chance of continuing.

When trying to make sense of different prognostic values associated with different ultrasound and biochemical findings as reported in different studies, remember that the background risk of miscarriage has a large influence on the prognosis of the current pregnancy. A patient with recurrent miscarriages, has a higher risk of miscarrying than your patient who conceived thanks to a stapled condom. Please also remember that the sizes that I quote, are averages and depend on the ultrasound unit, the ultrasonographer, and the patient.

The following are useful in assessing the early pregnancy prognosis:

Gestational sac

The first question is whether the gestational sac is intra- or extra-uterine. If it is intra-uterine, the risk of a heterotopic pregnancy (a simultaneous ectopic pregnancy) is miniscule. If no intra-uterine gestational sac can be seen, the pregnancy might be too early to be detected, or there might be an ectopic pregnancy. In this scenario, serial ßHCG-levels might help, in the following algorithm: No gestation sac seen on transvaginal scan and:
ßHCG above 1000 IU/l: possible ectopic
ßHCG below 1000 IU/l: repeat ßHCG after 48 hours:
ß

CG doubles; probably normal intra-uterine pregnancy; repeat transvaginal ultrasound after one week
ßHCG decreases: failing pregnancy; no further treatment needed, provided that ßHCG decreases to zero
ßHCG increases, but does not double: possible ectopic pregnancy
(The management of a possible asymptomatic ectopic pregnancy at this early gestational age, i.e. laparoscopically or medically, is a can of worms to be opened another day.)
The size of the gestational sac should correspond to the gestational age according to the probable date of conception or ovulation. Remember when the gestational age as calculated from the last normal menstruation (by your trusted ultrasound unit or the cornerstone of obstetrics, the gestational wheel) would be inaccurate:
Cycles significantly longer or shorter than 28 days (or, obviously, irregular menstrual cycles)
Oral contraception discontinued within the last three cycles (or, obviously, conception while using hormonal contraception)
The last menstruation was different (in volume and duration) from the usual menstruation.

Also look at the doughnut, not only the hole: the endometrium around the gestational sac should be thickened and echogenic; otherwise there is not much happening there to develop into a placenta, and without a placenta, a pregnancy cannot get very far.

Embryo and heart action
When the gestational sac is more than 25 mm, or the yolk sac more than 2,5 mm in diameter, an embryo should be seen. In an embryo longer than 2,5 mm, a heart action should be detected. (More strictly: if you can see the embryo, you should see the fetal heart action.) The following has a good prognosis, with more than 98% of pregnancies continuing:
A heart beat of monotonous regularity (the sympathetic and parasympathetic systems have not developed yet), with a biphasic signal (i.e. a shorter, sharper, higher spike as the heart contracts and empties actively, and a slower, lower wave as the heart relaxes and fills passively)
The following has a poor prognosis, with less than 25% of pregnancies continuing:
A weak signal (that does not trigger the doppler)
A monophasic pattern
Any arrythmia
A heart rate below 80/min after 7 weeks
A lack of growth (as evidenced by a crown rump length getting further behind dates) is also a poor prognostic factor.

Placenta and subchorionic haematomata

A placenta overlying the internal cervical os, triples the risk of significant bleeding from 10% to 30%, and doubles the background risk of a miscarriage from 8% to 15%. Where a subchorionic haematoma is seen, it is more dangerous the bigger it is, and the more centrally it is situated below the placenta (especially below the umbilical cord insertion).

Biochemical
Progesterone levels on their own are not worth much. A progesterone level above 30 mIU/ml is associated with a better pregnancy prognosis than one below this level. Whether to supplement low progesterone levels, is another can of worms. Sufficient to say that there are many, many more low progesterone levels caused by a failing pregnancy, that pregnancy failures caused by low progesterone levels.

In summary, a the following are associated with a good early pregnancy prognosis:
  • An intrauterine gestation sac
  • The gestational sac and yolk sac sizes are appropriate for the gestational age
  • A fetal heart signal is present, with a monotonous regularity of rhythm, and a biphasic pattern
  • The placenta does not overlie the cervi
  • There is no, or otherwise a small, peripheral subchorionic haematoma present.
  • The embryonic growth is normal
  • The ßHCG levels double within 48 h
  • The progesterone levels are above 30 mIU/ml
  • The following are associated with a poor prognosis:
  • An extrauterine gestational sac (or, no intra-uterine gestational sac is seen)
  • The gestational sac or yolk sac is too large or small for the gestational age
  • There is no heart action noticeable, or the heart action is weak or slow, or demonstrates an arrhytmia.
  • The placenta is overlying the internal cervical os.
  • There is a large subchorionic haematoma underlying the placenta.
  • There is poor embryonic growth.
  • The ßHCG levels plateau or decrease.
  • The progesterone levels are below 30 mIU/ml.