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A 25-year-old woman, gravida 1, para 0, at 12 weeks' gestation comes to the emergency department because of severe pelvic pain as well as the passage of bloody clots and tissues. She saw her primary care physician roughly four weeks ago and there were no abnormalities noted at the time. She has eaten a balanced diet and been compliant with taking prenatal vitamins. She denies recent fevers, trauma, or changes in physical activity. Pelvic examination shows a closed cervical os and the presence of blood within the vaginal vault, but the source of the bleeding is not visualized. Ultrasound shows a thickened endometrium but an empty uterus. Which of the following is the most likely cause of this event?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Pauline Rowsome, BSc (Hons), Tanner Marshall, MS, Sam Gillespie, BSc, Charles Davis, MD
Pregnancy is so complex that there are many different ways for the process to get off course and for a miscarriage to occur.
One of these problems is called aneuploidy, which is when there are missing chromosomes or additional chromosomes.
For example, if there’s one member of a chromosome pair missing, that results in 45 chromosomes total, instead of the normal 46, it’s called a monosomy.
And if there’s one extra chromosome joining a pair, that results in 47 chromosomes total, and it’s called a trisomy.
Some types of aneuploidy are viable like Turner’s syndrome or Down syndrome, whereas many are not and lead to a miscarriage.
Another abnormality is polyploidy, and that’s when a zygote receives more than one set of 23 chromosomes from either the sperm or egg, resulting in three sets, totaling 69 chromosomes, or even four sets, totaling 92 chromosomes.
Polyploidy is generally not viable and leads to a miscarriage.
One more abnormality is a translocation.
It can either be balanced, where two nonhomologous chromosomes essentially trade equal amounts of DNA, or unbalanced, where the chromosomes exchange unequal amounts of DNA, resulting in either too many or too few copies of certain genes on the involved chromosomes.
Now even if a parent carries a balanced translocation, the sperm or egg may end up with an unbalanced translocation, and the zygote won’t have a normal number of genes.
Some translocations are viable, whereas many are not, and can lead to a miscarriage.
Now, let’s say that there are a normal number of chromosomes present, and that the zygote becomes a blastocyst and tries to implant, there are still many ways in which a miscarriage can occur.
In the majority of cases, ectopic pregnancies won’t have the space or blood supply necessary to support a developing pregnancy.
As a result, the blastocyst stops growing and is reabsorbed.
If the blastocyst does implant in the endometrium and continues its development, it relies on the corpus luteum to maintain the pregnancy.
And one hypothesis is that a miscarriage can occur if the corpus luteum fails to secrete enough progesterone.
Finally, as the pregnancy continues and the placenta develops, the fetus begins to rely on the placenta to maintain the pregnancy.
If the placenta is unable to secrete sufficient levels of hormones or provide the fetus with a sufficient blood supply at any point, the fetus can stop growing and a miscarriage can occur.
During the embryonic period, which is between the third and eighth week of pregnancy, the fetus is especially vulnerable to damage from teratogens, which refers to anything that disturbs normal development – potentially resulting in a birth defect or death of the fetus.
Vital organs like the heart and brain are developing quickly during the embryonic period and teratogens can cause them to have severe abnormalities.
Between weeks 10 through 14 of pregnancy, chorionic villus sampling can be performed to assess the growing fetus.
That’s when either a needle or a catheter is used to grab a small sample of the placenta, so that a genetic analysis can be done on fetal cells from the placenta.
Around week 15, amniocentesis can be performed instead of chorionic villus sampling.
That’s when a needle is placed into the amniotic fluid, to pull out some fetal cells.
These procedures are usually well tolerated, but they both have a small risk of causing trauma and introducing an infection - both of which could lead to miscarriage.
As the fetus grows throughout the pregnancy, uterine abnormalities can also become an issue.