Government employees took to the streets of Bangkok on Tuesday to distribute 1 million baht's worth of prenatal vitamins containing folic acid and iron to women between the ages of 20 and 34.

The giveaway is part of a program called "the campaign for red-cheeked Thai women to have children for the country using very magical vitamins".

Thai couples are getting married less often and later in their lives, meaning fewer children each year.

"The Thai way of life is changing," said Dr Wachira Pengjuntr, director of the health department.

"Thai women are receiving higher education and the newer generation is placing a higher value on being single."

Thailand's population grew only 0.4% in 2015, down from 2.7% in 1970.

If this trend continues, the annual Thai population growth will be down to 0% in a decade, Dr Wachira said.