The increased incidence of PROM and pre-eclampsia in our twin sample verse the general population was of great interest. A literature review identified the incidence of PROM in the general population to be approximately 10% , whereas 24.3% of our pregnant women reported undergoing PROM in at least one of their pregnancies. Likewise, the incidence of pre-eclampsia in the general population is estimated to be about 4% , whereas 8.4% of our pregnant women reported having pre-eclampsia in at least one of their pregnancies. Since the number of pregnancies effected for each twin who had the condition was not known however, minimum and maximum odds ratios were calculated. Using this technique, the minimum odds ratio for PROM was 1.37 (95% CI = 0.95,1.96) and the maximum odds ratio was 5.42 (95% CI = 4.21,6.98). The actual odds ratio undoubtedly lies between these two numbers and is most likely significant.
Using the same technique the minimum odds ratio for pre-eclampsia was 1.17 (95% CI = 0.66,2.09) and the maximum odds ratio was 3.16 (95% CI = 2.16,4.62) in our twin sample verse the general population. Since both PROM and pre-eclampsia are more likely in subsequent pregnancies when one pregnancy has been affected, it is likely that at least some of the women in our sample were affected in more than one pregnancy. Interestingly, twin gestations themselves are associated with an increased risk of both PROM and pre-eclampsia. Whether there is a causative role behind the increased risk of PROM and pre-eclampsia during the antecedent twin gestation and subsequent pregnancies of these twin women remains unclear but is a potential area of further research. For instance, if twin women are more likely to develop pre-eclampsia than singleton women, aspirin prophylaxis for twin women may be routinely offered. Although there is wide variation on the usage of aspirin for pre-eclampsia prophylaxis, The American College of Obstetricians and Gynecologists (ACOG) does not currently list being a twin as an indication for prescribing aspirin for pre-eclampsia prophylaxis. Further, since the incidence of PROM and pre-eclampsia is known to vary across race, the racial breakdown in our sample needed to be similar to the general United States population for meaningful comparisons to be made. Overall, the socio-economic factors that make up this twin cohort are similar to the general United States population. The racial distribution of our twin sample was similar to the general population with 87.5% verse 78% identifying as White/Hispanic, 8.3% verse 12% identifying as Black, 2.1% verse 6% identifying as Asian, and 2.1% verse 4% identifying as other in our sample verse the general population respectively .
Another important consideration was the impact of smoking and PROM in our sample. Since smoking is a known cause of PROM it was very important to consider the potential role of tobacco use in our twin sample verse the general population. It has been estimated that in 2005, 13.8% of women reported smoking during pregnancy . In our twin sample 9.1% of pregnant women reported smoking at some point during their life. Whether the tobacco use in the twin women occurred during pregnancy or not was not specified so the actual percentage using tobacco during pregnancy was likely lower than 9.1%. Certainly, while smoking is often under-reported the smaller percentage in our twin sample verse the general population makes it unlikely that tobacco use played a significant role in our PROM findings.
The scant knowledge of amnionicity and chorionicity among twins was also striking. Although this is one of the most critical factors in the management and prognosis of twin gestations, less than 1 in 10 (9.9%) of twins sampled had this knowledge about their pregnancy. Interestingly, no fraternal twins (0/18) reported having this knowledge. However, every single twin who responded to this survey identified as either an identical or fraternal twin. Since 80 percent of all twins are dichorionic , and could thus be either identical or fraternal, the fact that 100% of the sample reportedly knew they were either identical or fraternal was curious. At any rate, there appears to be a general lack of knowledge about the embryology of twin pregnancies among twins themselves.
There were both strengths and limitations to this study. Strengths included a diverse group of female twins in terms of both age and demographics and completely voluntary participation in the study. The cross sectional study design allowed multiple obstetrical and gynecological parameters to be analyzed at the same time. Finally, the medical literature was useful in providing comparisons to our study sample. A significant limitation of the study was that there was no way to definitively corroborate that the twins took the surveys independently despite instructions to not collaborate. Unfortunately, this resulted in the possibility of twin collaboration and respondent bias. There was also a significant possibility of recall bias especially among older twins recounting obstetrical issues. The response rate was also lower than desired. Additionally, there were many non-statistically significant results and more identical twins than fraternal twins responded to the survey.