Implantation Timing After Fertilization
Implantation occurs approximately 6-12 days after fertilization, with the majority of successful pregnancies implanting between days 8-10. 1
Precise Timeline of Implantation
The window of implantation is well-defined based on direct measurement of human chorionic gonadotropin (hCG) appearance in maternal urine:
- Earliest implantation: 6 days post-ovulation 1
- Latest implantation for viable pregnancy: 12 days post-ovulation 1
- Most common timing: 84% of successful pregnancies implant on day 8,9, or 10 after ovulation 1
Clinical Significance of Implantation Timing
Later implantation is associated with significantly higher pregnancy loss rates:
- Day 9 or earlier: 13% early pregnancy loss 1
- Day 10: 26% early pregnancy loss 1
- Day 11: 52% early pregnancy loss 1
- After day 11: 82% early pregnancy loss 1
Conversion to Gestational Age
When calculating gestational age from the last menstrual period (standard obstetric dating), implantation occurs at approximately 3-4 weeks gestational age, since ovulation typically occurs around day 14 of the menstrual cycle and implantation follows 6-12 days later. 2
Important Clinical Context
The preimplantation period (fertilization to implantation) follows an "all-or-none" principle: exposure to teratogens during this window typically results either in failure of implantation/early miscarriage or complete recovery without malformation. 2 This is distinct from the organogenesis period (6-10 weeks gestational age), which carries the highest risk for structural malformations. 2
Human chorionic gonadotropin becomes detectable in maternal urine coinciding with implantation, making this the earliest biochemical marker of pregnancy. 1 Ultrasound visualization of a gestational sac typically occurs around 5 weeks gestational age. 2