What Every Pregnant Woman Should Know About Screening Ultrasounds
2nd-trimester screening ultrasounds can offer valuable information about your pregnancy!
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This post contains affiliate links. You can read our affiliate disclaimer at the bottom of this post.
1. Confirms Your Due Date:
Though a 1st-trimester ultrasound is most accurate at confirming your due date, a second-trimester ultrasound can still help to confirm the accuracy of your due date or EDC (estimated date of confinement).
Establishing your due date based on your last menstrual period is most accurate if your menstrual cycle is regular and ovulation occurs at midcycle.
However, if you have an irregular menstrual pattern (delayed ovulation, etc.) your due date based on the first day of your last menstrual period may not be correct.
A very rough rule-of-thumb regarding the accuracy of ultrasound for dating your pregnancy is as follows:
1st Trimester Ultrasound is accurate to within 1 week
2nd Trimester Ultrasound is accurate to within 2 weeks
3rd Trimester Ultrasound is accurate to within 3 weeks
The reasons an ultrasound becomes less accurate as your pregnancy advances are because the changes in growth parameters early in the 1st trimester compared to the 3rd trimester are less disparate.
It is also important to note that different ultrasound growth parameters are used with advancing gestational age.
A 1st-trimester ultrasound uses the crown-rump length of the developing embryo.
This measurement shows significant changes from one week to the next.
In contrast, a 3rd-trimester ultrasound is establishing a due date based on the measurements of a fully developed fetus.
The fetal head (biparietal diameter) is primarily used for dating and does not change significantly from week to week as compared to the weekly changes observed in the first trimester as previously noted.
Factor in a variation in the inherent size of a fully developed fetus and a potential growth-restricting factor and one can see why a due date based on a 3rd-trimester ultrasound is only statistically accurate to within 3 weeks.
With all this being said, it is important to keep in mind that the accuracy of an ultrasound due date will also depend on the quality of the imaging and the experience of the individual performing the procedure.
*** An easy way to get an estimate of your due date is to simply subtract 3 months from your last menstrual period and add 7 days. (This is based on a regular 28-day menstrual cycle.)
2. Detects Abnormal Placental Location (Placenta Previa)
Knowing the location of the placenta can be extremely important during your prenatal course for a number of reasons:
TYPES OF PLACENTA PREVIA
If you have a placenta previa (placenta overlying the cervix), this information can be extremely helpful to know, as this condition can result in painless vaginal bleeding, especially during the 3rd trimester.
There is no question that knowing you have a placenta previa (Complete, Partial or Marginal) can be invaluable information for both prevention of hemorrhage and clinical management (planning delivery) during the 3rd trimester.
The importance of knowing you have a placenta previa cannot be overstated.
The last thing your prenatal care provider would want to do examines your cervix if you have a placenta previa.
A cervical exam (in the presence of a placenta previa) can cause bleeding (potentially heavy vaginal bleeding).
For this same reason, bleeding can occur with intercourse.)
3. Singleton versus Multiple Gestation
When ultrasound became available, the days of being surprised by a twin pregnancy at the time of delivery essentially ended.
When I first started practice, this “story” was not unusual and was quite shocking for the expectant parent or couple at the time of delivery.
ULTRASOUND OF TWINS
This being said, the clinical findings of multiple gestation pregnancies are usually apparent by 20 weeks based on the uterine size being far greater than that of a singleton pregnancy. As a general rule, a singleton pregnancy will measure 20 cms at twenty weeks, with the uterus palpated at the level of the umbilicus.
A significant deviation from this (2 to 3 cms) should prompt an ultrasound evaluation to establish twins, triplets, etc..).
Once again, this information is extremely helpful to know as early in the pregnancy as possible.
Furthermore, knowing if the twins are in separate gestational sacs, etc. is crucial for optimal prenatal management.
4. Gender Determination
Though gender determination is exciting for expectant parents (who want to know the gender) this finding can also be clinically useful if there is a sex-linked genetic disorder within the family (ie hemophilia).
Knowing the gender can immediately resolve this concern and help prepare for neonatal management.
As noted above, gender determination in itself can be an exciting revelation as evidenced by the popular gender reveal parties that are now as popular as baby showers.
Though gender can be determined by ultrasound, there is a very small chance the findings could be incorrect.
However, with the level of ultrasound technology and expertise of ultrasonographers, gender determination using screening ultrasounds are now quite accurate.
The times when an imaging study can pose a problem are when the fetus is not in an optimal position for or if there is excessive fetal activity at the time of the ultrasound.
Usually, your ultrasonographer will be able to tell you their level of certainty based on the quality of the study.
As stated above, in most situations, the findings are quite apparent and often you will be given a picture at the time of the study.
Should genetic testing be required to screen for an inherited disorder, the gender results are far more accurate based on the chromosomes in the amniotic fluid.
5. Fluid Level
An ultrasound is a very useful way to measure the amniotic fluid around your baby. This information can represent different concerns depending on the gestational age of your pregnancy.
The measurement of amniotic fluid is another useful piece of information obtained during screening ultrasounds.
Low amniotic fluid is called oligohydramnios and can be seen with impaired placental function.
Increased amniotic fluid is called polyhydramnios and can be seen with diabetes during pregnancy, congenital anomalies.
6. Short Cervical Length or Preterm Dilated Cervix
Screening ultrasounds have created a large database of information about the cervix.
These often incidental findings can show anatomic changes that can help predict the risk of preterm delivery.
I mention this parameter simply because incidental findings such as this will continue to accrue as data is collected and these findings can be clinically significant when it comes to the prospective management of your pregnancy.
7. Uterine Abnormalities (congenital malformations, fibroids, etc.)
The prevalence of uterine structural abnormalities (be it fibroids or congenital malformations) is not uncommon and if not symptomatic, these structural differences which could impact pregnancy management will often go undiagnosed.
Once again, the question is simply whether it is helpful to know information of this nature when it comes to the management of your pregnancy.
There are those who might say “what you don’t know won’t hurt you” and though this may often be the case, the expression also comes to mind that “knowledge is power” or in this case can be instrumental in preventing pregnancy complications.
In additions to fibroids, uterine structural congenital anomalies, such as a bicornuate uterus may be found.
A bicornuate uterus can predispose to premature delivery and management can be affected if this finding is noted.
8. Serious Abnormalities That May Require Early Intervention
Though rare, I have seen incidental findings of concern on more than one occasion over the past 20 years. Abnormalities such as a molar pregnancy.
A molar pregnancy can give a “snow storm” appearance on ultrasound and be either partial or complete. Molar tissue can be cancerous.
Fortunately, molar pregnancies respond well to chemotherapy, often with complete remission. This is just one example of early detection leading to early intervention.
On physical exam (during your prenatal course) a molar pregnancy will often become clinically apparent.
However, this will not be apparent until the uterine size measures larger than the expected gestational age. (I want to reiterate that this condition is “rare” but not insignificant if it is you who is being affected by this medical condition.
9. Clinical Management Decisions
The management of your pregnancy can be affected base on information obtained from screening ultrasounds.
The following, are just a few examples:
- Decisions about intervention based on gestational age (prematurity or post dates)
- Determination of fetal growth abnormalities requiring antepartum management
- Delivery plans based on abnormal placental location (placenta previa)
10. Piece Of Mind
Though this list is not all-inclusive as to the benefits of screening ultrasounds, it is intended to explain how valuable the test can be during the early stages of your prenatal care.
Most important for an expectant mother or couple is the “piece of mind” screening ultrasounds can offer.
To simply know there are no major abnormalities identified on screening ultrasounds also reduces the possibility of finding a significant prenatal concern during the later stages of your pregnancy.
To summarize what screening ultrasounds can do for you:
- Placenta Previa and other forms of abnormal placentation (Placenta Accreta, Abruption, Twin Placenta abnormalities, etc.)
- Fibroids can be seen on screening ultrasounds
- Fetal Malformations and Syndromes
- Complications relating to Gestational Diabetes
- Gender determination (Hemophilia risk etc.)
- Confirm Due Date (this can be important to be certain of a diagnosis of preterm labor and post date management)
- Rare but serious pregnancy abnormalities (ie. molar pregnancy)
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