The beginning portion of what we consider the first month of pregnancy is actually your period until you ovulate. You are not actually pregnant during this time frame, but knowing the first day of your last normal period (LMP) we are able to more accurately calculate your due date.
Conception usually occurs within 24 hours of ovulation, about 14 days before the start of your next period. The egg and the sperm meet in the outer third of the Fallopian tube and travel over the course of the next seven to ten days, down the tube to the uterus. Once in the uterus, the fertilized egg will implant into the fluffy, nutrient rich lining of the uterus. Immediately chemical signals are sent to your body that pregnancy has occurred. Ovulation will halt. Your period will not come.
Your baby is called a blastocyst as it implants. The baby measures about 0.1-0.2 mm. At the end of the fourth week of pregnancy, the chorionic villi are formed. The yolk sac is helping to sustain the pregnancy until the placenta is fully formed. You might even begin to suspect you’re pregnant.