Side Effects

When should I expect my next period after I take emergency contraceptive pills?

You should have a normal period within the next month after taking
emergency contraceptive
pills (also called "morning
after pills" or "day after pills"). Sometimes,
emergency contraception
can change the length of your monthly menstrual cycle, making your
next period come as much as a week earlier or a week later than usual.
Some women also find that the hormones in the pills can cause unexpected
bleeding, but this is
not a common or serious side effect. If your next period is late,
you might consider getting a pregnancy
test.

Researchers have conducted three studies designed specifically to find
out how progestin-only
emergency contraceptive pills (like Plan B One-Step or Next Choice) affect women’s bleeding patterns.
Although the results were somewhat different, each found there were
often changes in a woman’s monthly menstrual cycle.

In the first study, women who took the full dose (1.5 mg levonorgestrel) of this emergency contraceptive at one time in the first three weeks
of their monthly menstrual cycles got their period much sooner than
expected – and earlier than women who did not take the pills.
The study also found that the earlier in their cycle women took the
pills, the sooner they got their period. Women who took the full dose
of emergency contraception
in the fourth week of their cycle got their period at the usual time,
but it lasted longer than normal. The ECPs had no effect on the duration of the post-treatment menstrual cycle, but the second period was prolonged. Intermenstrual bleeding was uncommon after ECP use, although more common than among women who had not taken ECPs.

The second study found that women who took the full dose (1.5 mg levonorgestrel) of this emergency contraceptive at one time within two days before or
after ovulation (when the egg is released) got their periods when
they expected them. But, on average, women started their periods a
day earlier when they used emergency
contraception more than two days before ovulation, and their periods
generally started two days later if they took the pills more than
two days after ovulation. Menstrual period duration increased significantly when ECPs were taken after three days before ovulation in both the treatment and post-treatment cycles. The duration of the post-treatment menstrual cycle remained significantly longer when ECPs were taken more than two days after ovulation. During the treatment cycle, 15% of women experienced intermenstrual bleeding; this was significantly more common when ECPs were taken in the preovulatory phase.

A third study examined the effects of two 0.75 mg levonorgestrel pills taken 12 hours apart. When taken in the follicular phase, ECPs significantly shortened the cycle when compared with usual cycle length; no effect on cycle length was found when ECPs were taken in the periovulatory or luteal phase. The post-treatment cycle length was the same as the usual cycle length.

A study comparing levonorgestrel (like Plan B One-Step, Next Choice One Dose, Next Choice or Levonorgestrel Tablets) and ulipristal acetate (ella) found that women taking ulipristal acetate had their next period on average 2.1 days later than expected, while women taking levonorgestrel began their next period 1.2 days earlier than expected, but the duration of periods was not affected.

More details on these studies and bleeding patterns overall can be
found in this thorough and up-to-date academic
review
of the medical and social science literature on emergency contraception.