If you become pregnant on mirena will it show up in a at home test? Or do you have to have a blood test?

The Mirena system may prevent sperm from reaching or fertilizing an egg. However, some sperm may reach the egg anyway, resulting in fertilization. When fertilization does occur, Mirena is also thought to act as birth control by preventing the embryo from implanting in the uterus. It is not known which of these mechanisms is most important for preventing pregnancy and most likely all of them work together. All mechanisms of the IUD are not completely understood.

Mirena IUS (http://www.contracept.org/iud-mirena.php)

The Minena Intrauterine System is thought to be very effective, with less than a 1% failure rate for perfect users. However, this number does not include women whose IUD fell out of place, which is a possible problem in the first few months after insertion. Mirena does not result in fertility problems for most users. After removing Minena, 20% of women trying to get pregnant were still unable to have a child a year after having Mirena removed. This is one reason that the manufacturers recommend Mirena only for women who have children.

Because no method is 100% effective, some women using Mirena will become pregnant. When this happens, one possibility is a septic abortion (infection in the uterus while pregnant), which is a leading cause of maternal death around the world. If pregnancy should occur with a Mirena IUS in place, it should be removed. This removal process may result in a miscarriage. However, not removing the device also increases the risk of miscarriage, as well as infection, premature labor and premature delivery. When pregnancy continues with Mirena in the uterus, long-term effects on the unborn child are unknown. Because synthetic hormones are being released right into the uterus, birth defects, and infant health problems are a possibility.