Most trisomy 21 miscarriages occur in the first trimester, particularly between weeks 7 and 12 of pregnancy. However, unlike many other chromosomal abnormalities that are almost always lost early, trisomy 21 pregnancies have a notable risk of loss well into the second trimester and even later, making it one of the few trisomies that can be lost at almost any stage.
First Trimester: The Highest-Risk Window
Chromosomal abnormalities are the leading cause of first trimester miscarriage, accounting for roughly 64% of all pregnancy losses when tissue is genetically tested. Between weeks 6 and 12, somewhere between 10% and 15% of all recognized pregnancies fail, and the rate of chromosomal error among those losses is high: around 48% at 7 weeks, dropping to about 37.5% by 12 weeks.
Trisomy 21 is one of the most commonly identified chromosomal abnormalities in first trimester miscarriage tissue, ranking just behind trisomy 16, which is the single most frequent trisomy found in early losses. In one large study of 406 miscarriage samples, 75% of the trisomy 21 cases came from women over 35, reflecting the strong influence of maternal age on this particular error. For comparison, trisomy 16 was more evenly split across age groups.
Why Trisomy 21 Losses Continue Beyond 12 Weeks
What makes trisomy 21 unusual among chromosomal abnormalities is that it doesn’t always cause loss early. Many trisomies (like trisomy 16) are so incompatible with development that virtually none survive past the first trimester. Trisomy 21 is milder in terms of developmental disruption, which means affected pregnancies often progress further before problems become critical. This is the same reason Down syndrome is one of the few trisomies compatible with live birth.
But “milder” doesn’t mean the pregnancy is unaffected. The placenta in a trisomy 21 pregnancy develops abnormally in several important ways. Cells that should fuse together to form the outer layer of the placenta often fail to do so properly. The placenta ages prematurely, showing signs of mineral buildup earlier than expected. One study found that about 80% of trisomy 21 placentas showed a pattern of blood vessel damage associated with stillbirth and growth restriction, compared to only 5% in chromosomally normal placentas. During the second and third trimesters, these placentas also show increased cell death in the tissue responsible for anchoring to the uterine wall, along with inflammation and underdeveloped blood vessels.
These placental problems accumulate over time, which is why trisomy 21 pregnancies remain vulnerable to loss long after the typical miscarriage window closes.
Second Trimester and Stillbirth Rates
Data from a large referral center tracking the natural progression of trisomy 21 pregnancies (those not electively terminated) found that 11% ended in second trimester miscarriage and 27% ended in stillbirth. The total fetal loss rate was 38%, meaning more than one in three trisomy 21 pregnancies that continued past the first trimester still did not result in a live birth.
These later losses are driven largely by the placental dysfunction described above. As the pregnancy demands more from the placenta in the second and third trimesters, a placenta that isn’t developing or functioning properly can’t keep up. Growth restriction is common, and in severe cases, the blood supply to the fetus becomes insufficient.
How Maternal Age Shifts the Risk
Maternal age is the strongest known factor influencing whether a pregnancy involves trisomy 21 in the first place. Among women under 24, about 14% of karyotyped pregnancy losses involve a trisomy of any kind. That proportion rises to approximately 38% in women between 40 and 44. Since trisomy 21 is one of the most common trisomies found in miscarriage tissue, older maternal age significantly increases both the chance of conceiving a trisomy 21 pregnancy and the chance of losing it.
This creates a pattern where women over 35 are disproportionately represented in trisomy 21 miscarriage data. In one study, 75% of trisomy 21 losses occurred in women over 35, a much higher age skew than seen with many other chromosomal abnormalities.
How Trisomy 21 Is Identified After a Loss
Trisomy 21 is typically identified through genetic testing of miscarriage tissue, also called products of conception testing. This involves analyzing cells from the placental tissue (chorionic villi) or fetal tissue to determine the chromosome count. The testing can be done through traditional karyotyping, which produces a visual map of the chromosomes, or through newer molecular methods like chromosomal microarray.
Not all miscarriages are tested this way. Genetic analysis is more commonly offered after recurrent losses or when the pregnancy was already being monitored for chromosomal concerns. In cases where testing is performed, trisomy 21 is one of the more frequently identified abnormalities, alongside trisomies 16, 22, 18, and 13. One case report documented a woman who experienced multiple miscarriages across several pregnancies, with chromosome testing on the tissue revealing trisomy 21 in several of the losses, including one at 14 weeks’ gestation.
If you’ve had a miscarriage and want to know whether a chromosomal abnormality was involved, tissue testing is the only definitive way to find out. It’s most reliable when tissue is collected and sent to a lab promptly after the loss, as cell viability decreases over time.