Are there regulations or official guidance regarding pregnant women flying ultralight aircraft at 6-7 weeks pregnancy (amenorrhea weeks)?
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.
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.
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.
There's nothing in regulations specifically about pregnancy, a pregnant woman can fly an airplane providing she is compliant with FAR 61.53, meaning that she would voluntarily "self ground" if she was unsure of her ability to fly.
Subpart A General Sec. 61.53 Prohibition on operations during medical deficiency.
(a) Operations that require a medical certificate. Except as provided for in paragraph (b) of this section, a person who holds a current medical certificate issued under part 67 of this chapter shall not act as pilot in command, or in any other capacity as a required pilot flight crewmember, while that person:
(1) Knows or has reason to know of any medical condition that would make the person unable to meet the requirements for the medical certificate necessary for the pilot operation; or
(2) Is taking medication or receiving other treatment for a medical condition that results in the person being unable to meet the requirements for the medical certificate necessary for the pilot operation.
(b) Operations that do not require a medical certificate. For operations provided for in Sec. 61.23(b) of this part, a person shall not act as pilot in command, or in any other capacity as a required pilot flight crewmember, while that person knows or has reason to know of any medical condition that would make the person unable to operate the aircraft in a safe manner.
(c) Operations requiring a medical certificate or a U.S. driver's license. For operations provided for in Sec. 61.23(c), a person must meet the provisions of--
(1) Paragraph (a) of this section if that person holds a valid medical certificate issued under part 67 of this chapter and does not hold a current and valid U.S. driver's license.
(2) Paragraph (b) of this section if that person holds a current and valid U.S. driver's license.
Pregnancy impacts women in different ways, and no two pregnancies are the same. At 6-7 weeks a woman would barely be showing, so control reach would not be a problem. The real issues come from the hormonal changes which can cause a variety of effects at that point. In rare cases women have not even realized they are pregnant at 6-7 weeks or even later, other women would be retching their guts out for almost their whole term.
Every pilot has to determine their medical fitness to fly every time the are going to sit in the cockpit, pregnancy is just one factor among many.