The Big Picture
Conception requires several precisely-timed events to align. An egg must be released from an ovary (ovulation), reach the fallopian tube, meet a viable sperm, and become fertilised. The resulting embryo must then travel down the tube, divide many times, and implant successfully in the uterine lining. Each of these steps depends on healthy hormones, anatomy and gametes. Even in healthy fertile couples, the chance of conceiving in any given month is only around 20-25% — which is why most couples take several months to conceive.
Step by Step
Ovulation: Around mid-cycle, hormonal signals from the brain trigger an ovary to release a mature egg into the fallopian tube. The egg lives 12-24 hours.
Sperm journey: After ejaculation, of the 200-300 million sperm released, only a few hundred reach the fallopian tube. Sperm can survive there for up to 5 days.
Fertilisation: One sperm penetrates the egg's outer layer and fuses with it, combining genetic material from both parents. This usually happens within hours of ovulation.
Cell division: The fertilised egg (zygote) starts dividing as it travels down the fallopian tube — into 2 cells, then 4, 8, 16, eventually forming a blastocyst by day 5.
Implantation: Around 6-10 days after fertilisation, the blastocyst attaches to the uterine lining and burrows in, beginning the maternal blood supply.
Hormonal signals: The implanting embryo produces hCG, the hormone detected by pregnancy tests. This signals the ovary to keep producing progesterone to support pregnancy.
Where Things Can Go Wrong
No ovulation (anovulation): Hormonal conditions like PCOS, thyroid problems or stress can prevent egg release
Tubal problems: Blocked or damaged tubes from infection, endometriosis or surgery prevent egg and sperm meeting
Sperm issues: Low count, poor motility or abnormal shape make fertilisation less likely
Egg quality: Particularly with age, eggs are more likely to have chromosomal abnormalities that prevent fertilisation or healthy embryo development
Fertilisation failure: Even when egg and sperm meet, they may not successfully fertilise
Implantation failure: Embryo may not attach successfully due to embryo quality, uterine lining or immune factors
Early loss: Many pregnancies end before they are even detected — these are sometimes called chemical pregnancies
Natural Conception Rates
For couples under 35 trying actively, around 80% conceive within 12 months
Around 20-25% chance of conception per cycle in your 20s
This drops to around 5% per cycle by age 40
Around 70% of conceived pregnancies result in a live birth
Most early pregnancy losses (chemical pregnancies and very early miscarriages) are due to chromosomal abnormalities in the embryo
Pregnancy is only possible during the 6-day fertile window each cycle
Even with perfect timing, conception is not guaranteed in any single cycle