Nexplanon can work immediately or take up to 7 days, depending on when in your cycle it’s inserted. If the implant goes into your arm during the first 5 days of your period, you’re protected right away. If it’s inserted at any other time, you need to use backup contraception like condoms for 7 days.
The 5-Day Window That Determines Everything
The key factor is where you are in your menstrual cycle on insertion day. Day 1 is the first day of your period. If your implant is placed between days 1 and 5, no backup birth control is needed. The timing works because you’re already in a phase of your cycle when pregnancy is extremely unlikely, and the implant’s hormone begins working before your next fertile window.
If the implant is placed on day 6 or later, it needs 7 full days to reliably prevent pregnancy. During that week, the hormone released by the implant (a type of progestin) is ramping up to effective levels in your body. It works by stopping ovulation, thickening cervical mucus so sperm can’t easily pass through, and changing the uterine lining. All three of those mechanisms need about a week to fully kick in when your body is already further along in its cycle.
Timing After Pregnancy or Abortion
The rules change depending on your situation. Here’s how the timeline breaks down for specific scenarios, based on CDC guidelines:
Not breastfeeding after delivery: If the implant is placed within the first 21 days after giving birth, it works immediately with no backup needed. After 21 days postpartum, the standard 7-day backup rule applies, because your body may be gearing up to ovulate again.
Breastfeeding after delivery: If you’re exclusively or nearly exclusively breastfeeding (at least 85% of feeds), haven’t gotten your period back, and are fewer than 6 months postpartum, no backup is needed regardless of when the implant is placed. Outside of those conditions, you’ll need 7 days of backup contraception.
After a miscarriage or abortion: If the implant is placed at the time of the procedure, it works right away. If it’s placed at a later date, use backup contraception for 7 days.
How the Implant Protects You
Nexplanon is a small, flexible rod about the size of a matchstick that sits just under the skin of your upper arm. Once in place, it steadily releases a progestin hormone into your bloodstream. This hormone does three things simultaneously: it prevents your ovaries from releasing an egg each month, it thickens the mucus at the opening of the uterus to block sperm, and it thins the uterine lining.
The combination of all three mechanisms is what makes the implant one of the most effective forms of birth control available. It’s over 99% effective, meaning fewer than 1 in 100 people using it will become pregnant in a given year. Unlike the pill, there’s nothing to remember daily, so the real-world effectiveness is essentially the same as the ideal-use effectiveness.
Does Body Weight Affect How Quickly It Works?
This is a reasonable concern, since the original approval trial for Nexplanon excluded people over 130% of their ideal body weight. That led to a label warning about potentially reduced effectiveness in heavier individuals. However, a more recent FDA-reviewed study that enrolled 152 overweight and obese participants found consistent results across all weight categories over a 2-year study period. The 7-day timeline for full effectiveness does not change based on your weight.
What Counts as Backup Contraception
During that 7-day waiting period, condoms (external or internal) are the go-to backup. You can also simply avoid vaginal intercourse during that week. Spermicide alone isn’t reliable enough to serve as your only backup. If you had unprotected sex during the 7-day window and are concerned about pregnancy, emergency contraception is an option.
Once those 7 days pass, or immediately if you had the implant placed during days 1 through 5 of your period, Nexplanon provides continuous pregnancy prevention for up to 3 years without any additional action on your part.