Pregnancy: implantation and positive pregnancy test

What is implantation? How does it occur? How does it relate to a positive pregnancy test? This article provides you with what you need to know about this phase of pregnancy.

Implantation: definition, signs and symptoms

Definition of implantation

Nidation is the stage that follows fertilisation. It is the moment when the embryo fully penetrates the lining of the uterus. Nidation begins on day 7 of fertilisation and lasts until day 10 of pregnancy. Therefore, the start of implantation is on days 21 and 22 from the last menstrual period.

Link between implantation and positive pregnancy test

During implantation, the egg, and more specifically its outer membrane, produces a substance called gonadotropic chorionic hormone (HCG). This hormone is detected by pregnancy tests.

At implantation, the HCG level exceeds 8 IU. It then doubles every 48 to 72 hours until the 2nd or 3rd month of pregnancy (between 7 and 12 weeks). The curve then reverses, and the HCG level gradually decreases until delivery.

Symptoms of implantation

Certain external signs indicate that implantation has occurred. For example, the pregnant woman may feel some abdominal pain, much like the cramps caused by menstruation. She may also experience slight spasms during the implantation period.

Between the 7th and 10th days after fertilisation, the pregnant woman may experience a discharge of blood called implantation spotting, which could be mistaken for a period. However, a difference exists between the two bleeds. The nidation bleeding is much more fluid and has a pinkish colour. Moreover, this bleeding lasts a much shorter time than menstrual bleeding. However, not all women bleed during implantation.

Other characteristics are associated with the implantation phase of the egg:

The HCG hormone and the pregnancy test

A positive pregnancy test detects the presence of HCG in the urine. This pregnancy test should be performed in the early morning hours as urine is more concentrated at this time. This urine test should be confirmed by a blood test. Once again, the HCG level in the blood determines whether the woman is pregnant. The dose of HCG in a pregnant woman's blood must exceed 5 mIU/mL or 5 IU/L.

The effects of HCG

The function of HCG is essentially the maintenance of the corpus luteum, which is responsible for secreting progesterone and oestrogen in the first 3–4 months of pregnancy.

The stages of implantation

These are segmentation, migration and the arrival of the embryo in the uterus.


Segmentation is a series of mitotic divisions beginning around 36 hours after fertilisation. This is when the first division occurs. It results in two identical blastomeres or cells. These two blastomeres divide into four other blastomeres and so on until, after 3 days, they reach the stage known as the morula or "little blackberry".


From the 3rd day, the embryo moves towards the uterine cavity, which it reaches on the 4th day of fertilisation. At this stage, the embryonic button can be clearly distinguished on one side and the trophoblast on the other.

Arrival in the uterus

Nidation describes the moment when the entire embryo enters the lining of the uterus. This key stage of pregnancy occurs from the 7th day after fertilisation; that is, the 21st or 22nd day after the last period, until the 10th day of pregnancy.

Ectopic pregnancy: abnormal implantation

On rare occasions, the fertilised egg fails to migrate normally into the uterine cavity. This is an anomaly known as an "ectopic pregnancy". In France, according to the 2011 report on the state of health of the population, 13,000 women were hospitalised in 2008 following the diagnosis of an ectopic pregnancy (EP). Specifically, 2% of pregnancies are affected by an EP, according to the same report, which also indicates that the number of ectopic pregnancies has doubled in the last 15 years. In 99% of cases of EP, the egg implants in one of the fallopian tubes; in other cases, it lodges in one of the ovaries or, rarely, in the peritoneum.

If a delay occurs in menstruation and bleeding (brownish blood flow) associated with localised pain in the lower abdomen, the doctor may suspect an EP if the pregnancy test is positive; 2/3 of cases of EP are treated by surgery and 1/3 by medication.