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Barbato M, Bitto A, Gray RH, et al. (June–September 1997). "Effects of timing of conception on birth weight and preterm delivery of natural family planning users". Advances in Contraception. 13 (2–3): 215–228. doi:10.1023/A:1006508106197. PMID9288339. S2CID24939823.
Gray, RH (October 1984). "Aged gametes, adverse pregnancy outcomes and natural family planning. An epidemiologic review". Contraception. 30 (4): 297–309. doi:10.1016/S0010-7824(84)80022-0. PMID6509983.
Gray RH, Simpson JL, Kambic RT (May 1995). "Timing of conception and the risk of spontaneous abortion among pregnancies occurring during the use of natural family planning". American Journal of Obstetrics and Gynecology. 172 (5): 1567–1572. doi:10.1016/0002-9378(95)90498-0. PMID7755073.
Barbato M, Bitto A, Gray RH, et al. (June–September 1997). "Effects of timing of conception on birth weight and preterm delivery of natural family planning users". Advances in Contraception. 13 (2–3): 215–228. doi:10.1023/A:1006508106197. PMID9288339. S2CID24939823.
Barbato M, Bitto A, Gray RH, et al. (June–September 1997). "Effects of timing of conception on birth weight and preterm delivery of natural family planning users". Advances in Contraception. 13 (2–3): 215–228. doi:10.1023/A:1006508106197. PMID9288339. S2CID24939823.