A wearable sensor apparatus is disclosed that includes a flexible substrate adapted to be coupled with a skin surface of an expectant mother. A conductor is disposed on the flexible substrate. The conductor can include micron-scale invaginations. The conductor can be capable of repeatable variation in resistance when subject to a strain. Also disclosed is a system for monitoring the health of a fetus in utero that includes a wearable sensor apparatus. The wearable sensor apparatus is configured to output a signal responsive to an electrical input. The system includes a computing system with one or more hardware processors. The computing system is programmed to implement a signal processing module configured to access the output signal from the wearable strain gauge and generate an output indicative of health of the baby in utero. The output can be based in part on the received output signal and previously stored correlations between signal data from the wearable strain gauge and observations of the system or of the mother. A user interface module can be provided and can be configured to display an output indicative of health of the baby in utero.