When does an embryo implant?

If you’ve ever wondered, “When does embryo implantation actually occur?” the answer lies within a carefully timed window. In humans, the implantation window typically occurs 7–10 days after an LH surge. For an individual, this window may be as short as 6 hours or as long as 24 hours within that timeframe. While pinpointing the exact moment of implantation is challenging, we know it’s guided by molecular signals that indicate when the uterine lining is most receptive to an embryo. Timing is crucial because the embryo must reach the mature blastocyst stage, where it has the ability to adhere to the uterine lining—both need to be ready for this "stick."

In cases of in vitro fertilization (IVF), timing is meticulously managed. Embryo transfers typically happen on the 5th day after egg retrieval or after 5 full days of progesterone injections. This timeline accounts for mimicking the natural LH surge, which occurs about 36 hours before egg retrieval and roughly 48 hours before progesterone reaches optimal levels. Essentially, the embryo is transferred about 7 days after mimicking the LH surge, aligning with the natural implantation window.