According to researchers, women use sense of smelling to choose the right kind of mate.

Previous
studies have shown that the body sends out aromatic molecules that
indicate genetic compatibility. Women generally chose men with
complimentary genes.

Researchers believe that offspring of
couples with different genes have better immunity system. It is also a
fact that couples with complimentary genes share a long lasting
relationship as compared to couples with similar genes.

The
recent research has shown that birth control pills can affect woman's
ability to choose right partner. Major histocompatibility complex (MHC)
genes are involved in immune response and other functions. According to
various studies, female generally choose male with dissimilar MHC smell.

Researchers
collected data collected from 100 women, aged 18 to 35. These women
were asked to choose from six male body-odor samples. These women were
not taking contraceptive pills in the beginning of the study.

The
women were again asked to choose from six male body odor samples.
During the span of three months, 40 women had started taking
contraceptive pills.

Researchers found out that there was no
significant change in preference of no-pill takers of MNC odors.
However, there was a drastic change in choice of women who started
taking contraceptive pills. The study showed that women taking birth
control pills preferred men with similar with MNC odors

Stewart
Craig Roberts said, "Not only could MHC-similarity in couples lead to
fertility problems but it could ultimately lead to the breakdown of
relationships when women stop using the contraceptive pill, as odor
perception plays a significant role in maintaining attraction to
partners."

He added that these pills put woman’s body in a
hormonally pregnant state. During pregnancy, the body doesn’t the feel
the need of a mate. He explained that when women are pregnant there's
no selection pressure. Hence they don’t have preference for genetically
dissimilar odors. He added that during pregnancy, women feel
inclination towards genetically similar individuals.