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In an attempt to stimulate further theory and research on only children, we introduce two conceptual distinctions: Only children as independent variables vs. only children as dependent variables; and only children by choice vs. only children by circumstances. What little scientific research exists on only children to date focuses almost exclusively on only children as independent variables and fails to make a distinction between only children by choice and only children by circumstances. A focus on only children by choice as dependent variables explores the question of why some women choose to have only one child. As an empirical illustration, analyses of prospectively longitudinal data with a nationally representative sample in the United Kingdom (National Child Development Study) show that women who experience pregnancy complications are significantly less likely to have another child and significantly more likely to have only one child. Our results suggest that increased chances of pregnancy complications that American women now experience alone can explain about 10% of the increase in the number of only children in the United States in the last half century. If certain genes incline women to have pregnancy complications, it is possible that only children by circumstances are genetically more similar to children with siblings than to only children by choice., Competing Interests: Declarations. Ethics Approval: Not applicable. There are no human or animal subjects involved. Consent to Participate: Not applicable. There are no human subjects involved. Consent for Publication: Not applicable. There are no human subjects involved. Conflict of interest: None. IRB Approval: IRB approval is not applicable in the current research, as no human or animal subjects were involved in the study., (© 2025. The Author(s), under exclusive licence to Society for Reproductive Investigation.)

There has been an increasing need for small, low-cost, and low-power consumption optical transceivers for short-reach fiber links. Waveguide-integrated photodetectors (PDs) with wide bandwidth and high responsivity on Si photonics platforms are an essential element for these applications. We have fabricated an O-band membrane PD which is suitable for integration with high-performance III-V-based membrane devices such as lasers and modulators, and passive waveguide circuits on the Si photonics platforms. The membrane PD consists of an InGaAsP-bulk absorption core embedded with an InP-based lateral p-i-n junction. The width of the InGaAsP absorption core is designed to be 350 nm and the doped regions overlap with the edges of the core to reduce the carrier transit time. In the structure, however, photocarriers generated in the doped core appear to be a limiting factor of the operating speed since the photocarriers diffuse into the depletion region with a long time constant. To avoid this, we design the bandgap wavelength of the absorption core to be shorter than the operating wavelength. As the electric field is mainly applied to the undoped InGaAsP absorption core region when the reverse bias is applied to the PD, the absorption coefficient in the undoped core region increases due to Franz-Keldysh effect (FKE) while the doped regions maintain a low absorption coefficient due to the wavelength detuning. A membrane PD with an absorption length of 30 µm integrated with the SiO x waveguide was fabricated by using a heterogeneous integration technique on a Si photonics platform. We verified that the FKE causes the photocurrent to increase. In addition, the OE bandwidth was observed to increase due to the wavelength detuning. The fabricated PD exhibited a fiber-to-PD responsivity of 0.6 A/W and a bandwidth over 67 GHz. Eye openings for 100-Gbit/s NRZ signals were demonstrated at a stage temperature of 25°C.