To answer this question we firstly go back to basics to understand your cycle.
The pregnancy is calculated based on the average menstrual cycle which is 28 days. The first day of menstrual bleeding marks the first day of the menstrual cycle. This is called your LMP (or “last menstrual period”). Right in the middle of this picture perfect 28 day cycle is when ovulation occurs (day 14 of the cycle) and roughly 2 days before and after is when you are most fertile. In real life not everyone has this picture perfect 28 day cycle, it may be longer or shorter, but we know that ovulation occurs roughly 14 day before your next period. This is why many fertility doctors recommend to have intercourse every 3 days if you really are trying to have a baby because the ovulation may occur a little earlier or later than you may think.
When calculating the weeks of pregnancy we include those two weeks from the first day of the LMP as this is the easiest part of the cycle to notice and to record. We also will enquire about the length of your cycle and whether it is regular or irregular. All of this is helpful in trying to establish the gestation. For example, if your cycle is regular and 33 days long we would expect your ovulation to have occurred later (5 days) and an early pregnancy scan will most likely show a baby that measures 5 days less than you were expecting.
Now to answer your question, it may be that you ovulated later in the cycle or have a longer cycle. The pregnancy is truly only 5 weeks, a follow up ultrasound in 10-14 days is likely to show your baby’s heartbeat and all is well. The other possibility is that unfortunately the pregnancy is not developing as expected at 7 weeks gestation and a follow up scan may reveal a miscarriage. We understand that this is an anxious time and to have to wait for 10 -14 days seems very long but it is important to have this time in between scans to determine accurately what is taking place.