So fortunately for you I brought home my copy of Jenny Jochens's Women in Old Norse Society and didn't leave it in my pile of books at work.
Unfortunately for the both of us, this quite thorough work on women in the Norse world is really quite silent on the kinds of birth control that we are accustomed to hearing about. While there are historical precedents for contraceptives such as physical barriers, herbs, and so on, they go unremarked on in the source material that Jochens is drawing from. This really isn't surprising. The sagas and legislation that form the core of our textual evidence of life pre-conversion (and keeping in mind their limited utility) area already rather skimpy when it comes to talking about women in period. I'll leave it to the amazing /u/sunagainstgold to talk about why this is, and for all the information on this topic I'm sure we could ever wish for.
History is full of different methods of controlling fertility. Physical barriers such as condoms are known, certain herbs that could force a miscarriage are also pretty well attested, changing sexual positions, certain exercises, amulets, spells, and the list goes on. The efficacy of each method obviously varies to a certain degree.
Now, that is not to say that there is no evidence for what happened in the cases of unwanted pregnancies specifically among the Norse(whether it be by the mother or some outside person). Jochens supposes, based on later Christian prohibitions and condemnations (and some limited support early on after conversion) that the exposure of infants was a quite common, or at least well known practice. Leaving the infant to the "mercies" of the wild is a well known practice in many societies across the world and the Norse were seemingly quite well versed in it.
Jochens supposes that this practice originated as the husband, or at the very least father, choosing whether or not to accept the child, or in lieu of a known father, the woman's male relatives. If the child was not accepted or not believed to have been the man's offspring the child would then be left in the wilderness, where excepting for survivors in the sagas, it is likely they met their end. While there are no hard statistics on this practice, Jochens supposes that it was more common to happen to daughters or in economically insecure households.
This practice was strenuously prohibited following conversion to Christianity, despite some early tolerance in Iceland. In Anglo-Saxon England, the penalty for infanticide was 15 years penance, and this likely held true for the Norse areas as well given the importance of the English church in Scandinavian conversion. The Gragas, an Icelandic law compilation, decrees that all children born shall be brought to be baptized, no matter their deformity. This mirrors similar developments in Norway as well, though Jochens indicates that in some extreme cases of deformity exposure was still allowed. Later in time this would no longer be the case, and in Norway infanticide carried the same penalties as murder.
Later law codes continue to stress the importance of a prompt baptism and in a healthy dose of sexism are extremely concerned with establishing that the mother did not murder her own child, indeed witnesses to stillbirths are supposed to check for signs of foul play.
As for later, or earlier in the case of older societies, there is a great deal more evidence about the forms of birth control utilized. I know that I've focused on infanticide to the exclusion of other forms of contraception, but it is quite simply the only one dealt with in depth by the sources of the day. This is not the case for later in the medieval period however, but it does hold true for the Viking age.
Sources:
I've used mostly Jenny Jochens's Women in Old Norse Society