Signs and symptoms
Generally it is preferable to describe specific signs in lieu of declaring fetal distress that include:

Decreased movement felt by the mother
Meconium in the amniotic fluid ("meconium stained fluid")
Non-reassuring patterns seen on cardiotocography:
increased or decreased fetal heart rate (tachycardia and bradycardia), especially during and after a contraction
decreased variability in the fetal heart rate
late decelerations
Biochemical signs, assessed by collecting a small sample of babys blood from a scalp prick through the open cervix in labor
fetal metabolic acidosis
elevated fetal blood lactate levels (from fetal scalp blood testing) indicating the baby has a lactic acidosisSome of these signs are more reliable predictors of fetal compromise than others. For example, cardiotocography can give high false positive rates, even when interpreted by highly experienced medical personnel. Metabolic acidosis is a more reliable predictor, but is not always available. Complications
Complications are primarily those associated with insufficient fetal oxygenation, most notably increased mortality risk. Other complications include fetal encephalopathy, seizures, cerebral palsy, and neurodevelopmental delay. Causes
Several conditions and risk factors can lead to fetal distress or non-reassuring fetal status, including:

Low amniotic fluid (oligohydramnios)
If there is too little amniotic fluid around the baby in the uterus, the baby can have trouble moving around in the uterus and its growth and temperature can be impacted. Low amniotic fluid can be caused by placental issues, high gestational blood pressure, some medications, as well as problems with the fetal kidney or urinary tract.
Staging
Surgically, endometriosis can be staged I–IV by the revised classification of the American Society of Reproductive Medicine from 1997. The process is a complex point system that assesses lesions and adhesions in the pelvic organs, but it is important to note staging assesses physical disease only, not the level of pain or infertility. A person with Stage I endometriosis may have a little disease and severe pain, while a person with Stage IV endometriosis may have severe disease and no pain or vice versa. In principle the various stages show these findings:Stage I (Minimal)

Findings restricted to only superficial lesions and possibly a few filmy adhesions.Stage II (Mild)

In addition, some deep lesions are present in the cul-de-sac.Stage III (Moderate)

As above, plus the presence of endometriomas on the ovary and more adhesions.Stage IV (Severe)

As above, plus large endometriomas, extensive adhesions. Implants and adhesions may be found beyond the uterus. Large ovarian cysts are common. Markers
An area of research is the search for endometriosis markers.In 2010, essentially all proposed biomarkers for endometriosis were of unclear medical use, although some appear to be promising. The one biomarker that has been in use over the last 20 years is CA-125. A 2016 review found that this biomarker was present in those with symptoms of endometriosis; and, once ovarian cancer has been ruled out, a positive CA-125 may confirm the diagnosis. Its performance in ruling out endometriosis is low.