Conception vs implantation: what’s the difference?

Conception and implantation are often used as if they mean the same thing, but they happen about a week apart, in different places, and only one of them can show up on a pregnancy test.

Conception

Conception, or fertilisation, is when a sperm joins with an egg. It happens in a fallopian tube, within about a day of ovulation. Because sperm can survive for up to about 7 days, conception can happen several days after you had sex.

At this point the fertilised egg is a single cell. It carries all the genetic information for your baby, including their sex. Nothing measurable changes in your body yet: there’s no hormone a test can find, and you won’t notice any symptoms.

The journey in between

Over the next few days, the fertilised egg divides again and again as it moves slowly down the fallopian tube. By about 5 days after fertilisation it has become a blastocyst, a hollow ball of cells. Some of those cells will become your baby, and others will become the placenta.

Implantation

Implantation is when the blastocyst reaches your womb and attaches to its lining, burying into it. This happens around 6 to 12 days after ovulation, most often 8 to 10 days. Once it’s implanted, the cells that will become the placenta start making hCG, the hormone pregnancy tests detect.

The key differences

When: conception is around ovulation; implantation is about a week later.
Where: conception happens in the fallopian tube; implantation normally happens in the womb.
Signs: there are none at conception. Around implantation some people notice light spotting or mild cramps, though it isn’t certain implantation causes them.
Tests: a test can’t pick up a pregnancy until after implantation, once hCG has built up.

Why it matters

It explains why you can’t test straight after sex, and why tests are most reliable from the day your period is due. It also explains why pregnancy is counted from your last period: nobody knows the exact moment of conception, but most people know when their period started.

Not every fertilised egg implants, and not every implanted pregnancy carries on. A pregnancy that stops very early can show briefly as a positive test before a period arrives.

If a fertilised egg implants outside the womb, usually in a fallopian tube, it’s called an ectopic pregnancy. It can’t continue and needs medical care, which is why one-sided pain or bleeding in early pregnancy should always be checked.

Working out when you conceived

If you have regular 28-day cycles, you probably conceived about 2 weeks after the first day of your last period, or about 38 weeks before your due date. If your cycles are longer or shorter than 28 days, add or take away the difference. With a 35-day cycle, for example, it’s more likely to be about 3 weeks after your period started.

Your dating scan, usually between 10 and 14 weeks, measures your baby and gives the most accurate due date. If it moves your due date, your likely conception date moves with it. Even then it’s an estimate, because sperm can survive for several days, so the day you had sex and the day you conceived may not be the same.

Conception and implantation in IVF

In IVF, fertilisation happens in the lab, and the embryo is usually transferred to the womb 2 to 5 days later. Implantation then happens within a few days of the transfer, which is why your clinic gives you a test date based on your transfer rather than your period.

Questions people ask

Are you pregnant at conception or at implantation?

Medically, a pregnancy is dated from your last period, and a test can only confirm it after implantation. In everyday terms, most people think of themselves as pregnant once they get a positive test.

Are twins decided at conception?

Non-identical twins come from two eggs fertilised by two sperm, so they’re decided at conception. Identical twins happen when one fertilised egg splits into two, usually within the first couple of weeks.

Can you feel either?

You can’t feel conception. Some people notice spotting or mild cramps around implantation, but many feel nothing. See can you feel conception? and can you feel implantation?

When to get help

Mild cramps that ease with rest are usually nothing to worry about. But if you’re pregnant, or think you could be, get urgent advice from your maternity unit, an early pregnancy unit or NHS 111 if you have:

any bleeding or spotting, even if it’s light
tummy pain that’s severe, on one side, or hasn’t eased after resting for 30 to 60 minutes
pain in the tip of your shoulder

Call 999 if you have bleeding along with severe tummy pain, pain in your shoulder, or feeling sick, faint or dizzy, or if the bleeding is heavy (soaking a pad soon after putting it on). Also call 999 if you have sudden, sharp and intense tummy pain and you feel very dizzy or faint, feel sick or look very pale. These can be signs of an ectopic pregnancy or a miscarriage that need treating straight away.