Ultrasound: Placenta and amniotic fluid
Does the ultrasound scan show anything else apart from the fetus?
Yes. Scanning in pregnancy is not confined to looking at the fetus. In the second half of the pregnancy, an ultrasound scan will be used to establish the location of the placenta, to check the amniotic fluid volume and to look at other parts of the pelvis and abdomen - especially the ovaries and kidneys.
Why is placental location important?
Because some placentas may be low-lying. This can have potential bearing on the timing as well as the method of delivery.
A condition known as placenta praevia - which means "low-lying placenta" - could cause vaginal bleeding during pregnancy. This is sometimes serious enough to require an emergency delivery.
Even without bleeding, when the placenta is significantly low-lying, it means delivery will most likely be by caesarean section. Diagnosis by ultrasound scan allows for proper pregnancy care and planning of the time and method of delivery.
Why is amniotic fluid volume measurement important?
Abnormalities of amniotic fluid volume (either too much or too little) sometimes denote a problem with the pregnancy.
An ultrasound scan allows the doctor to make note of the fluid abnormality, its extent and its progression. More importantly, it allows for the initiation of necessary steps, either to diagnose the underlying cause or to monitor the fetal well-being.
Does a low-lying placenta at 18-20 weeks denote placenta praevia?
No. At this stage, anything between 30 and 40 per cent of placentas appear low-lying. Less than 1% will still be low-lying towards the end of pregnancy.
The placenta itself does not move. It is the pattern of growth of the womb that allows for the placenta to appear further away from the lower segment as the pregnancy advances. There is no such thing as placenta praevia at 20 weeks of gestation.
Ultrasound: Different stages of pregnancy
How accurate is the ultrasound at estimating gestational age?
Apart from the first seven weeks or so, the earlier the scan is performed, the more accurate it is. In the first trimester (the first fourteen weeks), the scan will be accurate to within three days of the gestation. In the middle of the second trimester (at about 20-22 weeks), the accuracy drops to about a week. If the scan is performed in the third trimester, it may miss the gestation by up to three or even four weeks. The bigger the fetus, the more inaccurate the scan tends to be.
Generally speaking, scans performed in the last ten to twelve weeks of pregnancy cannot be used to date a pregnancy because their margin of error is considered too wide.
If I had an ultrasound scan in the early part of the pregnancy which established the gestational age, then a repeat scan several weeks later showed a different "gestation", isn't this confusing?
It should not be. Your earliest scan is the most accurate one. A subsequent scan should not prompt a re-calculation of the gestation age. Once this is established, it does not change, no matter what.
Why would there be a discrepancy between scans of the same fetus performed at different phases of the pregnancy?
Because of the inherent weakness of the ultrasound in dating late pregnancies, the differing growth rates of babies and, not least, human error. The emphasis here is that if a scan is to be useful in dating a pregnancy, this has to be done as early as possible, preferably below twelve weeks and certainly before twenty weeks.