Valentine S. Steshenko, a Soviet demographer from the Ukraine, arrived in Cambridge Monday night as part of a visit to learn what American researchers are doing in the field of population studies.

Steshenko, a specialist in statistical studies of fertility among women in the Ukraine, is now studying the effects of socioeconomic variables on the rate of reproduction.

The fact that the Ukrainian birthrate is presently below the new replacement rate is of special concern to her. The best solution, she said, is to provide more and better day care facilities "so mothers will regard the day care centers as even better than home, and will have more children while they continue to work."

The most surprising feature of demographic research here, she said, is the interdisciplinary approach Harvard has toward population studies. The staff at the Ukrainian institute is comprised solely of professional demographers, whereas Harvard's population center is a composite of experts from different disciplines.

Steshenko is head of the Department of Theoretical and Applied Research at the Ukrainian Academy of Sciences, and is travelling on a four month UNESCO fellowship to the US and England.