How Implantation Works: Days 6 to 12

In numbers

• MeasuredImplantation: 6–12 days after ovulation, most often 8–10
• MeasuredFully buried in the lining by about day 10
• DrawnhCG is drawn as light; it is invisible

Every number is labelled measured, an estimate or drawn: how we know.

The science

The blastocyst attaches by its embryonic pole, usually high on the back wall of the uterus. Where it touches, the trophoblast divides into an inner cytotrophoblast, still made of separate cells, and an outer syncytiotrophoblast, one mass with many nuclei, which invades the stroma of the lining. By about day 10 the conceptus is fully embedded and a plug of fibrin seals the way in. Lacunae open in the syncytiotrophoblast and, as it erodes the mother's capillaries, make first contact with her blood. Plugs of trophoblast cells keep her spiral arteries closed for most of the first trimester, so real blood flow into the placenta begins only around weeks 10–12. Meanwhile the inner cell mass becomes a disc of two layers, epiblast and hypoblast, and the amniotic cavity opens between the epiblast and the trophoblast. The syncytiotrophoblast secretes hCG, which keeps the corpus luteum making progesterone. In a study that measured hormones every day, implantation (timed by the first hCG in urine) came 6–12 days after ovulation, most often on days 8–10. Many pregnancies end around this time, most before anyone knows.

In Endo-View

This is chapter 9 of the journey to life, an interactive 3D simulation at true scale: watch it, slow it down or run it at real speed, and read every number with its label.

Watch it in 3D