I. Terms related to pregnancy interval 1. interpregnancy interval (IPI): also known as the birth-to-pregnancy (BTP) interval, refers to the time interval between live birth and the start of the next pregnancy. 2, Interval between births (IDI): also known as birth between two births (BTB), refers to the time interval between consecutive live births. Because BTB does not take into account miscarriage and fetal death, the BTB interval calculation may be the same for both women, even if one woman has only 2 pregnancies during this interval and the other has multiple pregnancies. 3. pregnancy outcome interval (IOI): the time interval between two pregnancy outcomes, regardless of the specific pregnancy outcome. Because the IOI counts all pregnancies, it provides a better risk assessment for stillbirth, spontaneous abortion, or induced abortion. 4. too short/long IPI: The definitions of too short and too long IPI have not been standardized. too short IPI is defined as less than 3, 6, 9, 12 or 18 months. an IPI of less than 6 months is most often associated with poor outcome. too long IPI usually refers to an IPI of more than 60 months. Risk factors for short IPI 1. According to the maternal depletion hypothesis, if two pregnancies are closely spaced, the mother’s nutrients (especially folic acid) may not be adequately replenished during the second pregnancy, especially in breastfeeding mothers, and this may lead to poor pregnancy outcomes. Studies have reported a higher risk of fetal growth restriction in women with low puerperal serum folate levels and a short IPI who did not use folic acid supplements. The incidence of preterm premature rupture of membranes (PPROM) and preterm birth (PTB) may be increased in women with a short IPI in a second pregnancy if the previous pregnancy was characterized by inflammation of the reproductive tract and was not completely cured. 3. Other factors that may contribute to the association of short IPI with poor obstetric and neonatal outcomes include cervical insufficiency, sibling competition for maternal resources (e.g., lactating pregnancy), transmission of infection between siblings with a short birth interval, and incomplete healing of the uterine scar from a previous cesarean delivery. III. Risk factors for prolonged IPI “physiological regression hypothesis”: pregnancy causes important physiological adaptations in the reproductive system with a time frame, such as increased uterine blood flow, and these changes eventually disappear and are not present in women with prolonged IPI. IV. What is the optimal pregnancy interval? The optimal IPI is unclear and may depend in part on the outcome of the previous pregnancy. 1. After a full-term live birth: Both WHO and USAID recommend that IPI should be greater than 2 years and less than 5 years after a full-term live birth. The WHO recommends an IPI of 2 years between delivery and conception, which is consistent with the UNICEF recommendation of 2 years for breastfeeding. 2. Advanced maternal age: For women of advanced age, an IPI of only 12 months may be more reasonable, as it balances the progressively increasing risk of low fertility and infertility with age against the increased risk of pregnancy complications (including maternal death or serious complications) associated with a very short IPI (<6 months). However, the literature on the interaction between IPI and advanced maternal age is limited. The risk of low fertility and infertility with age must be weighed against the risk of maternal and perinatal complications associated with a too short IPI on a case-by-case basis. 3. IPI in women undergoing assisted reproductive technology: In a study of patients undergoing assisted reproductive technology (ART), an interval of less than 12 months after delivery to the start of ART treatment was associated with an increased risk of preterm delivery and low birth weight in singleton live births. Therefore, these data support that the initiation of in vitro fertilization (IVF) should be delayed after live birth until at least 12 months have elapsed. 4. After spontaneous or induced abortion: Couples who wish to conceive after a spontaneous abortion may not necessarily have improved outcomes if they follow the conventional recommendation to wait 6 months or more before conceiving, so they may begin conception whenever they are ready. In another study, if the IPI is less than 7 months after spontaneous abortion, the mother has an elevated risk of anxiety or depression in the next pregnancy. 5. After a stillbirth: Women who have had a stillbirth can begin conception at any time when they are ready. Some clinical guidelines recommend waiting 15-24 months; however, at least two large studies have found that the risk of recurrence of stillbirth, although elevated, is not affected by the time between pregnancies. 6. After cesarean delivery: There are some special considerations after cesarean delivery. Uterine rupture associated with TOLAC can be very serious, including causing fetal and maternal death, and significantly increases the risk associated in further pregnancies. Women considering TOLAC should be informed about the risks associated with a too short IDI so that they can better plan for another pregnancy. We recommend a longer IDI (18-24 months) because a longer IDI is not significantly associated with maternal complication rates among women undergoing TOLAC. 7. post-eclampsia: Women with an IPI less than 12 months are at higher risk of recurrent pre-eclampsia than women with a longer IPI. Therefore, we recommend that IPI should be 18 months, except for special cases, such as advanced maternal age. 8. After preterm birth: Women with IPI less than 6 months have the highest risk of preterm birth. Therefore, it is recommended that IPI should reach 18 months, except for special cases (advanced maternal age).