The chances of conceiving or getting pregnant while using any kind of contraceptive method might be less but not nil. This is a reality that you must be prepared for while using any kind of birth control method.
Conceiving with an IUD:
If you have an IUD (intrauterine device/coil) and still conceive, then the chances of a miscarriage are greatly increased. Apart from the high spontaneous abortion rate, there is also a risk of developing adverse gestational and neonatal complications. The chances of an ectopic pregnancy are also higher if you have an IUD. It is best to consult an obstetrician as soon as you become aware of your pregnancy. The IUD must be removed at the earliest date possible and is ideal to be done during the early days of pregnancy. Many obstetricians advise termination if you and your partner are not planning for a baby. But termination is definitely not necessary if you wish to have the child and are aware of the possible complications.
Your obstetrician may remove the IUD by pulling at the strings if it is visible or resort to removal by ultrasound guidance if this is safe at that particular point of pregnancy. Never try to remove the IUD yourself and consult a doctor at the earliest.
Sometimes, if it becomes too late to remove the IUD and you want the baby, the doctor may advise you to go ahead with the pregnancy. There are many women who have uneventful pregnancies and childbirth with an IUD in which case the device is delivered along with the placenta. However, if you are pregnant with the IUD inside, then it is advisable to be monitored regularly to ensure that the baby is fit and healthy.
Getting Pregnant while on the Pill:
Even if you have been taking the pill, there is a small chance that you can conceive. The chances are more on the days you forget to take the pill or suffer from a stomach upset and vomiting in which case the hormones do not get absorbed or if you are taking antibiotics. If you have conceived and without being aware of it continue to take the pill, then the chances of your baby being born with congenital abnormalities are slightly higher. But on the other hand, there are women who have been on the pill while pregnant but give birth to normal and healthy babies. The fact is that there is a slightly small but definitive risk. Many studies have reported a risk factor though clinical experience has been otherwise.
Most women do not realize that they are pregnant while on the Pill because the signs and symptoms of pregnancy like breast tenderness, nausea and irregular menstrual bleeding can also be side effects of being on the pill especially if you are taking the minipill (birth control pill with only progestin). If you conceive while on this pill, the chances of ending up with an ectopic pregnancy are higher.
If you think you are pregnant while on the Pill, then the best thing would be to stop using the pill and consult a doctor to confirm if you are pregnant or not. During this time, you can use natural birth control methods like condoms till you have ruled out or confirmed pregnancy.