Pregnancy begins with a series of carefully timed events in the body. Understanding them can help couples trying to conceive know when they are most fertile and what happens in the early days and weeks.
How pregnancy happens, step by step
1. Ovulation
Each month, hormones cause an egg to mature in one of the ovaries. Around the middle of the cycle, the egg is released and picked up by the fallopian tube. The egg can be fertilised for about 12 to 24 hours. Learn about the phases of the menstrual cycle.
2. Fertilisation
During intercourse, millions of sperm are released, but only a few hundred reach the fallopian tube. Sperm can survive for up to five days, so intercourse in the days before ovulation can lead to pregnancy. When one sperm enters the egg, fertilisation happens and the genetic material of both parents combines.
3. Cell division
The fertilised egg, now an embryo, divides repeatedly as it travels along the fallopian tube towards the uterus over about five days. By day five or six it becomes a blastocyst, a ball of cells with an inner group that will form the baby and an outer layer that will form the placenta.
4. Implantation
About six to ten days after ovulation, the blastocyst attaches to the lining of the uterus. It starts producing the hormone hCG, which is detected by pregnancy tests.
The stages of pregnancy
First trimester (weeks 1 to 12)
The baby’s main organs begin to form, and the heart starts to beat at around six weeks. Many women have nausea and tiredness. See common pregnancy symptoms.
Second trimester (weeks 13 to 27)
The baby grows quickly, and most women feel movements between about 18 and 22 weeks. A detailed anomaly scan is usually done around 20 weeks.
Third trimester (weeks 28 to 40)
The baby gains weight and the lungs mature. Labour usually starts between 37 and 42 weeks. See pregnancy and delivery care.
How pregnancy is dated
Pregnancy is counted from the first day of the last period, about two weeks before conception. A full-term pregnancy lasts about 40 weeks from that date.
When conception does not happen
Most couples conceive within a year. If you have been trying for longer, or six months if you are 35 or older, see when to see a fertility specialist.
Frequently asked questions
How long after intercourse does fertilisation happen?
Fertilisation can happen within hours of intercourse if an egg is present, or up to five days later, because sperm can survive in the reproductive tract.
When does implantation happen?
About six to ten days after ovulation.
When can a pregnancy test detect pregnancy?
Most home tests are reliable from the day a period is due. Blood tests can detect pregnancy a few days earlier.
If you have questions about your own situation, you can meet our doctors at our Basavanagudi, Koramangala and Ballari centres, or contact us to book an appointment.
This article is for general information and does not replace medical advice. Treatment decisions should be made with a qualified doctor who knows your history.