Heartburn and breathlessness in late pregnancy have a mechanical explanation rather than a mysterious one. The available space inside the abdomen has changed, and so has the tone of certain muscles.
Space is the limiting factor
The uterus expands upward through the abdomen, displacing the stomach, intestines and diaphragm from their usual positions. Nothing is removed; everything is compressed.
Stomach capacity effectively falls, which is why large meals become uncomfortable and smaller frequent ones are tolerated better.
The displacement increases through the third trimester and often eases slightly when the baby's head settles lower in the pelvis toward the end.
The valve at the top of the stomach relaxes
A ring of muscle at the junction between oesophagus and stomach normally prevents contents moving upward. Pregnancy hormones relax smooth muscle throughout the body, including there.
Reduced tone in that ring, combined with upward pressure from below, allows acid to reach the oesophagus. The lining there has no protection against it.
This is why heartburn worsens lying down and after eating. Both increase the pressure differential across a valve that is already less competent than usual.
Digestion slows throughout
The same smooth muscle relaxation slows movement through the whole digestive tract, which increases the time food remains in the stomach.
Slower transit contributes to constipation, another common late feature, and to a sense of fullness disproportionate to the amount eaten.
The slowing is thought to increase nutrient absorption by extending contact time, which suggests the discomfort is a side effect of something useful.
Breathlessness starts earlier than the bump explains
Breathlessness often appears in the first trimester, before any mechanical pressure exists. Hormonal changes alter the sensitivity of the breathing control system.
The result is deeper breathing and an increased awareness of it, which is experienced as breathlessness despite oxygen levels being entirely adequate.
Later on, the raised diaphragm reduces how far the lungs can expand downward. The chest compensates by widening, but the sensation of restriction remains.
What is not part of the ordinary picture
Breathlessness that comes on suddenly, occurs at rest, or is accompanied by chest pain, a fast heart rate or coughing is not the mechanical version and needs urgent assessment.
Severe or persistent upper abdominal pain, particularly on the right, is assessed separately in pregnancy because it can indicate a different condition.
Ordinary heartburn is common and management options exist, but which are suitable in pregnancy is a question for a midwife, pharmacist or doctor rather than general reading.