The journey to life · chapter 2
How Sperm Get Through the Cervix: Cervical Mucus
The cervix is the narrow passage into the womb, and it is filled with mucus. For most of the month the mucus is thick and sperm cannot get through. Around ovulation it turns thin and watery, and its strands line up into channels. Strong swimmers follow the channels; slow and misshapen sperm get stuck. Some hide in small pockets in the cervix and wait there, for days.
- Where
- Cervix
- How long
- Minutes, up to 5 days
In numbers
- EstimateSperm can survive in the body for up to about 5 days
- MeasuredMid-cycle mucus is about 93–98% water
- EstimateGetting through: roughly 1 in 100
Every number is labelled measured, an estimate or drawn: how we know.
The science
Cervical mucus is a mesh of mucin glycoprotein strands, about 93–98% water at mid-cycle. Near ovulation, oestrogen makes it thin, stretchy and aligned: its strands line up into lanes, though the spaces between them are under a micrometre, smaller than a sperm's head, and sperm push them apart. Sperm are found in cervical mucus within minutes of ejaculation. Cervical crypts store sperm and release them slowly. That is why intercourse up to 5 days before ovulation can still lead to pregnancy. Only around 1% of sperm make it into the cervix.
In Endo-View
This is chapter 2 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.
The other chapters
What Happens to Sperm After Ejaculation Sperm in the Uterus: The Race and the White Blood Cells Sperm in the Fallopian Tube: Waiting for the Egg The Human Egg Cell: Its Size and Its Layers How Fertilisation Works: Sperm Meets Egg The Zygote and the First Cell Divisions What Is a Blastocyst? Days 3 to 6 How Implantation Works: Days 6 to 12 The Fetus from Week 10 to Week 20 Pregnancy Week by Week: The Fetus to Week 40 How Birth Works: Labour and the First Breath The Embryo Week by Week: Weeks 4 to 10
More of the body
The heart and blood vessels The lungs and airways The digestive system The kidneys and the urinary system The brain and the nervous system