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Pregnancy loss during the second trimester occurs between the 14th and 23rd week of gestation. This occurs in around 1-2% of all pregnancies. Research has shown that the main reasons for fetal loss during the second trimester include chorioamnionitis, funisitis, and the placenta being infected with a virus. One could expect that chromosome abnormalities would be another major cause. This is not the case because most fetuses that have a major chromosome abnormality don't make it to the 14th week of gestation .
Women who have had a previous miscarriage during the second trimester have an increased risk of another miscarriage compared to the general population. It is suggested that the maternal uterine environment and abnormal placentation are the main causes of poor pregnancy outcome. This could be due to lesions and or inflammation of the placenta, placental abruption (separated from the uterus of the mother) or hemosiderin (a protein that stores iron in the body). To help test for the potential of having a poor pregnancy outcome, serum screening can be done. The maternal serum screening can be helpful because it has been established that lesions of the placenta can be shown in the chronic migrating choriodeciduitis and acute choriodeciduitis .
Current research is being done to figure out ways to examine the placenta, and to understand the pathology, to help provide clinical insight into the causes of the pregnancy losses during the second-trimester. Figuring out the relationship between poor second-trimester pregnancy outcomes and gaining a better understanding of abnormal placental pathology will go a long ways to understanding how second trimester pregnancy losses (and preterm births) occur .
The rate of second-trimester pregnancy loss is lower in women around 20 to 30 years of age. Black women have a higher risk of having a second-trimester pregnancy loss .
Amniocentesis is another risk factor for fetal loss during the second trimester. Women who are around or greater than 40 years of age have a higher risk for fetal loss after having an amniocentesis done compared to those aged 20-34 years. The risk factors for fetal loss after an amniocentesis for those >40 years of age inlude a history of first trimester abortions, a history of second trimester miscarriages, and bleeding during the current pregnancy .
This research shows that there are age-specific fetal loss (spontaneous) rates during a specific time during gestation. The spontaneous loss of pregnancies has been shown to occur at or after 15 weeks of gestation .
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 Srinivas SK, et al. 2008. Can placental pathology explain second-trimester pregnancy loss and subsequent pregnancy outcomes? Am J Obstet Gynecol 199(4):402.e1-5
 Wyatt PR, et al. 2005. Age-specific risk of fetal loss observed in a second trimester serum screening population. Am J Obstet Gynecol 192(1):240-6
 Papantoniou N E; Daskalakis G J; Tziotis J G; Kitmirides S J; Mesogitis S A; Antsaklis A J. 2001. Risk factors predisposing to fetal loss following a second trimester amniocentesis. BJOG : an international journal of obstetrics and gynaecology 108(10):1053-6.