The first signs and symptoms of pregnancy generally emerge three weeks after fertilisation, but in some cases the pregnancy already shows signs the sixth day after conception. Thus, despite not being common, it is perfectly possible that the woman already has symptoms during the first days of the pregnancy.
The signs and symptoms that lead to suspecting a pregnancy include:
Amenorrhea (lack of menstruation/period)
Nausea and vomiting
Breast pain (mastalgia)
Frequent and urgent micturition
Constipation, fatigue and weight increase
It is important to point out that the signs and symptoms of pregnancy are not the same for all women. Furthermore, the same woman can have completely different symptoms between two different pregnancies. The signs of pregnancy can also vary in their intensity, frequency and duration.
A period during which the woman and the fetus will experience a series of physical and psychological changes. In this link you can see the evolution week by week.