Implantation is a critical early stage in every pregnancy.
This article explores the signs that implantation has happened. It also reviews a few false signs of early pregnancy and suggests when to take a pregnancy test.
An egg is fertilized by a sperm in one of the two fallopian tubes. These connect the uterus with the ovaries, which sit to either side.
The fertilized egg is called a zygote, and it travels down the tube toward the uterus. As it moves, it divides and grows, becoming a multicellular structure called a blastocyst.
The first stage of implantation occurs when the blastocyst attaches to the lining of the uterus, a mucous membrane called the endometrium. The blastocyst then buries itself in the uterine wall.
Implantation is a complex process with a brief window — it happens about 6–10 days after ovulation.
One factor influencing the timing is that the endometrium changes, thickening and shedding, throughout the menstrual cycle. It is only receptive to a blastocyst for a few days in each cycle.
Even if the timing is right, implantation may not happen. According to researchers, a lack of implantation is a common cause of early pregnancy loss.
Indications that implantation has happened can include bleeding and cramping.
Implantation bleeding is spotting or light bleeding that takes place about 1–2 weeks after fertilization. It is usually short-lived and lighter than a normal period. The blood is often paler than menstrual blood or can be a rusty brown.
There has been little research about implantation cramping, but it may affect the lower abdomen or back and feel like:
- light pulling
Early pregnancy can cause additional symptoms, which may start a short time after implantation and continue for the first trimester and sometimes beyond. They can include:
- feeling very emotional
- vomiting, especially in the mornings, often from around 6–9 weeks
- a headache
- having enlarged or tender breasts, sometimes with the nipples sticking out
- needing to pee more frequently
- a strange taste in the mouth
- food cravings and aversions
- changes in weight
Some pregnant people experience many of these symptoms, while others experience none.
A timeline of implantation and early pregnancy includes the following events, based on a 28-day menstrual cycle:
- Day 1: the first day of a period
- Day 14: ovulation
- Day 15: fertilization — the egg can be fertilized for about 24 hours after ovulation
- Day 20–24: implantation of the blastocyst in the uterus
The following symptoms can indicate implantation, but they can also have very different causes, including pregnancy loss.
Vaginal bleeding occurs during 15–25% of early pregnancies, according to The American College of Obstetricians and Gynecologists. It can be a sign of implantation, but it can instead point to:
- a miscarriage
- an ectopic pregnancy
- bleeding from the cervix, due to increased blood flow to the area
- an infection
Also, bleeding sometimes happens due to a pelvic examination, sex, or a Pap smear.
Cramping or lower back pain can indicate implantation, but it can also occur with early pregnancy loss.
Because pregnancy tests are now so sensitive, some people know when they have experienced a very early pregnancy loss, known as a chemical pregnancy. The loss occurs at just around 5 weeks.
Also, a person might confuse implantation bleeding and cramping with a period, though any symptoms of implantation usually take place a week or so before a period is due.
Often, light bleeding, spotting, and cramping in early pregnancy resolve with no problems.
However, anyone experiencing bleeding or more serious pain should consult a doctor. These may be, for example, symptoms of an ectopic pregnancy, which is a medical emergency.
In addition, pregnant people with rhesus-negative blood may need an injection of rhesus immune globulin after any bleeding, to prevent the fetus from developing health issues such as hemolytic anemia, according to the National Heart, Lung, and Blood Institute.
Home pregnancy tests detect the hormone human chorionic gonadotropin (hCG) in urine.
The pregnant body starts producing this hormone before implantation. Levels in the blood and urine double about every 24 hours for 8 weeks, peaking at about 10 weeks.
Home pregnancy tests can detect hCG in the urine about 12–15 days after ovulation, in people with 28-day menstrual cycles.
A positive pregnancy test result may be possible on the day a missed period is due. However, the tests are not so reliable that a negative result at this stage definitely means that the person is not pregnant.
The American Association for Clinical Chemistry suggest that in this case, a person should test again about 10 days after the period was due.
Implantation is a complex process essential to the establishment of a pregnancy.
It is sometimes accompanied by spotting or light bleeding, and the blood may be pale pink or rusty brown. Some people experience cramping.
However, bleeding and cramping can indicate a range of issues, including pregnancy loss, and are not a reliable indicator of implantation.
For greater certainty, use a home pregnancy test at least 10 days after a missed period, though some tests can detect pregnancy earlier, on the first day a period was due.