Combination pills contain both estrogen and progestin, two types of hormones similar to those made naturally by a woman's ovaries. This combination pill works primarily by preventing the ovaries from releasing an egg each month. Some combination pills, like Yas and Ortho Tri-Cyclen Lo, contain less estrogen for women who feel the effects of the hormone more strongly or those who prefer to take minimal doses. The mini-pill, which only contains progestin, works by thickening a woman's cervical mucous and thinning the lining of the uterus. It may also suppress ovulation, but not reliably like combination pills. For this reason, the Mini-Pill is less forgiving than the combination pill and MUST be taken at the same time each day.