Is it safe to fly ultralights at 6-7 weeks pregnancy?
Ultimately the decision of whether or not a pregnant woman should be piloting an aircraft is between her and her doctor, and you shouldn't take the commentary of strangers on the internet as medical advice ("I am not a doctor, and even if I were I am not your doctor").  

That said while I'm not sure what EASA has to say on the subject I know the FAA does not consider pregnancy to be a "disqualifying medical condition" for operations requiring a medical certificate, so it would logically not be disqualifying for operations which do not require a medical certificate (gliders, balloons, or ultralights) either.



Ultimately much of the advice out there comes down to "you can fly through the second trimester, and shouldn't fly in the third trimester" - assuming a normal and uncomplicated pregnancy, with the arguments against flying in the third trimester largely focusing on the onset of labor, obstruction of the flight controls, or difficulty with the aircraft's restraints (seatbelts).

Some specific concerns that have been identified for pregnant pilots include:


Inability to get full range of movement on the flight controls
If the pilot's "baby bump" is large enough to obstruct the free movement of the flight controls they probably shouldn't be flying an airplane.
This is a concern for all pilots with larger-than-average abdomens, but as pregnant women tend to enlarge quickly it's something they need to be aware of on each flight.
Onset of labor
All the women I know who have given birth have lead me to believe it would be rather difficult to simultaneously fly an aircraft and give birth, so it would be sensible to avoid single-pilot operations (including ultralight flying) late in pregnancy where this might be an issue.
Blood Clots
Sitting in the same position for long periods of time increase the chance of blood clots. This is a problem for all pilots, but pregnant women can be more susceptible to blood clots so flights should be kept short if possible.
Hypoxia
Mom is breathing for two, and physiologically the fetus has a priority claim on oxygenated blood. As a result pregnant women may require oxygen at lower altitudes, but this is not likely to be a concern for ultralight pilots as you won't be climbing that high.