What it is
Saptamsha divides each sign into seven equal parts. Thirty degrees divided by seven gives about 4°17′ per part. Odd and even signs start their count from different places.
How it is calculated
Rule: in an odd sign (Aries, Gemini…) the first part belongs to the sign itself, then onward in order. In an even sign (Taurus, Cancer…) the first part belongs to the 7th sign from it, then onward.
Example: a planet is at Aries 17°. 17 ÷ 4°17′ ≈ 3.97, so it is in the fourth part. Aries is odd, so we count from Aries: Aries, Taurus, Gemini, Cancer. In D7 the planet is in Cancer. At Taurus 17° the count would start from Scorpio and the fourth part would be Aquarius.
What tradition looks at in it
Parāśara links the saptamsha to sons and grandsons, that is children and the next generation. Tradition looks at the D7 ascendant, the lord of the 5th house and Jupiter, the traditional significator of children. It is one way of looking at the theme of a growing family and one's bond with children.
How much the birth time matters
On average the ascendant moves about 1 degree every 4 minutes; some signs rise faster, some slower. One part of D7 is about 4°17′, so a difference of roughly 17 minutes in birth time can change the D7 ascendant. The Moon crosses one part in about 8 hours. In your chart, ePandit shows next to every varga how many minutes of birth-time change would shift its ascendant. Even hospital-recorded times can be off by a few minutes, so read the D7 ascendant with care.
How to read it calmly
Read D7 as a second look, not a verdict. Tradition first studies the 5th house and its lord in the main chart (D1), then the running dasha. D7 only helps check whether the same theme shows up here as well. A planet's sign in this varga decides nothing on its own. For any health matter around children or pregnancy, a doctor is always the first support. A chart never replaces medical advice.