Party-goers are being urged to remember to practise safe sex during the festive season.
The warning came from family planning charity Marie Stopes International after a survey showed that 23% of women aged 16 to 24 have had unprotected sex after drinking too much alcohol.
The poll of 503 women also showed that four in 10 have used the morning after pill at least once as emergency contraception.
The charity urged women in particular to think about taking extra precautions.
It said, for example, young women could consider using long term contraception methods such as implants, the coil or injections, all of which should be used with a condom to prevent sexually-transmitted infections.
Tracey McNeill of Marie Stopes International said that compared with using the pill or a condom alone, these methods could help to reduce unplanned pregnancies.
She added: "Unfortunately, Marie Stopes International experiences a significant increase in women accessing our unplanned pregnancy services during the New Year as a direct result of men and women forgetting to use or having their contraception fail over the Christmas and New Year festivities."
The charity said that its clinics see a rise of 10% in women wanting abortions in the months January to March, which are their busiest months of the year. It said that this was as a result of contraceptive failure over the Christmas period.