Contraception would be very important in space, because of limited medical resources for pregnancy and potentially hazardous/deadly effects on embryo (such as increased radiation exposures). No method other than sterilization is 100% effective; the effectiveness of oral contraceptive in microgravity is not known. In microgravity, sperm may linger in vaginal tract, as their movement is not dependent upon gravity: transport by muscle contractions, ciliary activity, and the motility of sperm. On the other hand, microgravity and spaceflight by themselves may have contraceptive effects. Menstrual dysfunction is likely due to disturbance in circadian rhythms, intensive exercise, stress. Disturbance of the hypothalamic-pituitary-ovarian axis lead to lack of ovulation or excessive menstrual bleeding. This may include retrograde menstruation, producing endometriosis and infertility. Male fertility: fall in testosterone level and sperm motility, exposure to toxins used in life-support and propellants could reduce sperm counts.
Higher radiation levels can affect both male and female fertility. Sperm cells are the most radiosensitive in human body, resulting in reduced fertility or genetic abnormalities. Sperm cells produced on a 74-day cycle, so levels return to normal after low radiation exposures. Radiation doses needed to destroy all sperm cells are usually fatal. Ovaries lie 5-7 cm below the skin, so some slight shielding. Oocytes more radio-resistant to genetic defects than are sperm cells, but are not replaced if damaged. In women, a radiation exposure of LD50 (enough to produce 50% fatality rate) results in sterilization (destruction of all oocytes and end to estrogen production) but effects of radiation are cumulative. Radiation can cause endometriosis.
Conception has occurred in female rats in space, but the embryos were reabsorbed. Male rats mated after flight produced offspring with higher rates developmental abnormalities, including growth retardation. But fish produced viable offspring. The opverall effects of space environment on human fertility unknown.
Pregnancy in space. Must consider pre-existing pregnancy or one originating in space. Adverse effects (microgravity, radiation) are expected to be greatest during the early stages, particularly genetic damage to to the embryo. Morning sickness would be more difficult to manage, if combined with Space Adaptation Syndrome. This might lead to "hyperemesis gravidarum" with dehydration and liver damage. A woman might experience all the complications of a pregnancy on Earth, combined with limited medical care, which could lead to serious injury or death; conditions like urinary tract infections and gestational diabetes would be more difficult to treat. Example: pre-eclampsia - hypertension, protein in urine, headache, visual disturbances, can worsen into eclampsia, with potentially fatal seizures and coma. Hemorrhagic complications: abruptio placentae - limited blood supplies for transfusion, hypovolemia and mild anemia could worsen effects; fetus more sensitive to maternal blood loss - anoxic encephalopathy.
Other complications: ectopic pregnancy, pre-term labor, rupture of the amniotic sac during acceleration/deceleration of launch and re-entry, exposure to toxins.
Might there be benefits to microgravity? It could reduce incidence of varicose veins, edema, back pain and difficulty in moving in later pregnancy, lightheadedness due to compression of the inferior vena cava when supine.
Childbirth in microgravity would require securing the woman and her assistants, maintain sterility, and containing fluids (such as amniotic fluid); spinal anesthesia is partly dependent upon gravity; cesarean section would require higher level of sterility, trained personnel, and proper equipment, than is currently available in space.
Decompression sickness: nitrogen bubbles in the circulatory system of a fetus can pass from veins into arteries through foramen ovale and ductus arteriosus (fetal bypass), would be much more dangerous if they occurred in arterial side and would impair blood supply to critical organs, such as brain and heart. Other risks to fetus: radiation danger is extreme (maximum allowed adult dose for 9 months is 0.5 rem, in LEO - Low Earth Orbit - annual exposure is 14-21 rem). If in deep space or if there is a solar particle event, the resulting exposure can be very high. If this occurs in the first 2 weeks of pregnancy, it would result in the destruction of the zygote. Developmental abnormalities would occur from exposure during 8-25 weeks gestation.
The probability of conception with effective contraception is low. However, the risk to the mother and developing embryo/fetus is very high. Risks are increased with time in space and distance from terrestrial medical care. Problems may be ultimately surmountable but for now, the risk/benefit is very unfavorable.
Kids in space: children are more susceptible to radiation than are adults, and the risks of microgravity are also worse. Bones/muscles still developing, so microgravity is likely to result in growth retardation, especially the long bones in thighs and legs. Also possible are delayed closure of the fontanels, short stature, abnormal development of vertebrae, potentially resulting in nerve compressions syndromes. Epiphyses may close early. Adults need vigorous exercise to maintain 85% muscle/bone mass, but kids can't use adult-sized equipment and may be uncooperative, might result in levels well below 85%.
They may experience difficulty learning to walk and balance, and problems with neurovestibular reflexes. Effects on overall neurological development are unknown.
Long-term radiation exposure increases lifetime risk of cancer. Immature immune system may increase risk of infection. Just having small children in spacecraft is recipe for disaster.
Currently, sex in space is both risky and potentially deadly. But we must find ways to reduce these risks for successful long-term flight and eventual colonization of space.
Sex with aliens requires a bare minimum of "complementary" anatomy. Also faith that alien sexual practices do not lead to unexpected consequences, confusion of expectations if species has more than 2 genders, black-widow-spider syndrome. Offspring very unlikely even with genetic engineering. Alien physiology almost certainly radically different from terrestrial, evolved under different condition, different DNA, proteins, amino acids, cells, etc.