📚 The 280-Day Pregnancy | 280天妊娠
The average human pregnancy lasts approximately 280 days (40 weeks) from the first day of the last menstrual period. This timeline is a central concept in reproductive biology and provides a framework for understanding fertilisation, implantation, embryonic development, and birth.
人类妊娠平均持续约280天（40周），从末次月经第一天算起。这一时间线是生殖生物学的核心概念，为理解受精、着床、胚胎发育和分娩提供了框架。
1. Fertilisation and Implantation | 受精与着床
Fertilisation occurs in the ampulla of the oviduct (fallopian tube) when a sperm nucleus fuses with an egg nucleus. The resulting zygote undergoes mitotic divisions as it travels towards the uterus.
受精发生在输卵管壶腹部，当精子核与卵子核融合时。形成的受精卵在向子宫移动过程中进行有丝分裂。
Around day 5 after fertilisation, the embryo develops into a blastocyst, which attaches to the endometrial lining of the uterus. Implantation is completed by day 14 of the menstrual cycle, marking the beginning of pregnancy.
受精后约第5天，胚胎发育成胚泡，附着在子宫的子宫内膜上。着床一般在月经周期第14天完成，标志着妊娠的开始。
- The zona pellucida prevents polyspermy during fertilisation.
- 透明带在受精过程中防止多精受精。
- The blastocyst secretes human chorionic gonadotropin (hCG) to maintain the corpus luteum.
- 胚泡分泌人绒毛膜促性腺激素（hCG）以维持黄体。
2. The Placenta | 胎盘
The placenta is a disc-shaped organ formed from both fetal and maternal tissues. It allows the exchange of oxygen, nutrients, and waste products between the maternal and fetal blood streams without direct mixing.
胎盘是由胎儿组织和母体组织共同形成的盘状器官。它允许氧气、营养物质和废物在母体血液和胎儿血液之间交换，而不直接混合。
Blood flows through the umbilical artery to the placenta, where carbon dioxide and urea diffuse into maternal blood. Oxygen and glucose diffuse in the opposite direction into fetal blood, returning via the umbilical vein.
血液通过脐动脉流向胎盘，二氧化碳和尿素扩散进入母体血液。氧气和葡萄糖则以相反方向扩散进入胎儿血液，并通过脐静脉返回。
| Structure | Function |
| Villus | Increases surface area for diffusion |
| Umbilical cord | Connects fetus to placenta |
| Placental barrier | Prevents mixing of maternal and fetal blood |
胎盘绒毛增加表面积，脐带连接胎儿与胎盘，胎盘屏障阻止母体与胎儿血液混合。
3. Embryonic Development Stages | 胚胎发育阶段
During the first 8 weeks after fertilisation, the developing organism is called an embryo. Organogenesis, the formation of organs, occurs during this period. The heart begins beating around day 22, and limb buds appear by week 4.
受精后前8周，发育中的生物称为胚胎。器官形成（器官发生）在此期间进行。心脏在第22天左右开始搏动，第4周出现肢芽。
From week 9 until birth, the organism is called a fetus. During the fetal period, growth and maturation of systems dominate. The lungs mature late, and the brain undergoes major development in the final trimester.
从第9周到出生，该生物称为胎儿。在胎儿期，主要是系统的生长和成熟。肺部成熟较晚，大脑在最后三个月经历重要发育。
Key milestones include: week 12 (sex can be distinguished), week 20 (movement felt by mother), week 24 (lungs begin producing surfactant), and week 28 (fetus has a good chance of survival if born prematurely).
重要的里程碑包括：第12周（可分辨性别）、第20周（母体感到胎动）、第24周（肺开始产生表面活性物质）、第28周（早产存活率较高）。
4. Fetal Circulation | 胎儿循环
The fetal circulatory system differs from the postnatal system. Oxygenated blood from the placenta travels through the umbilical vein, partially bypasses the liver via the ductus venosus, and enters the inferior vena cava.
胎儿循环系统与出生后不同。来自胎盘的含氧血液通过脐静脉，部分经静脉导管绕过肝脏，进入下腔静脉。
Blood flows from the right atrium to the left atrium through the foramen ovale, and from the pulmonary artery to the aorta through the ductus arteriosus. These shunts allow most blood to bypass the non-functional lungs during fetal life.
血液通过卵圆孔从右心房流向左心房，并通过动脉导管从肺动脉流向主动脉。这些分流使大部分血液在胎儿期绕过未发挥功能的肺。
- Umbilical vein: carries oxygenated blood from placenta to fetus.
- 脐静脉：将含氧血从胎盘运往胎儿。
- Umbilical arteries: carry deoxygenated blood from fetus to placenta.
- 脐动脉：将缺氧血从胎儿运往胎盘。
After birth, the foramen ovale closes and the ductus arteriosus becomes the ligamentum arteriosum, directing blood to the lungs.
出生后，卵圆孔关闭，动脉导管退化为动脉韧带，引导血液流向肺部。
5. Maternal Changes During Pregnancy | 妊娠期母体变化
The mother's body adapts to support the growing fetus. Blood volume increases by about 40-50% to provide adequate circulation to the placenta. Cardiac output rises, and respiratory rate increases slightly.
母体身体适应以支持生长中的胎儿。血容量增加约40-50%，以便为胎盘提供充足循环。心输出量增加，呼吸频率略有加快。
The uterus expands from about 50 g to over 1 kg by term. Hormonal changes cause relaxation of smooth muscle, which can lead to constipation and heartburn. The kidneys also work harder to excrete fetal waste via the mother's blood.
子宫从约50克扩张到足月时约1千克。激素变化引起平滑肌松弛，可能导致便秘和胃灼热。肾脏也加倍工作，通过母体血液排泄胎儿废物。
Typical maternal weight gain ranges from 10-15 kg, including the fetus, amniotic fluid, placenta, increased breast tissue, and fat stores.
母体体重增加通常在10-15千克之间，包括胎儿、羊水、胎盘、乳腺组织和脂肪储备。
6. Hormonal Regulation | 激素调节
Pregnancy is maintained by a complex interplay of hormones. After implantation, trophoblast cells secrete human chorionic gonadotropin (hCG), which rescues the corpus luteum from degeneration and ensures continued production of progesterone and oestrogen.
妊娠由多种激素相互作用维持。着床后，滋养层细胞分泌人绒毛膜促性腺激素（hCG），使黄体免于退化，并确保持续产生孕酮和雌激素。
From the second trimester, the placenta itself takes over steroid hormone production. Progesterone maintains the endometrial lining, suppresses uterine contractions, and helps develop the placenta. Oestrogen promotes uterine growth and stimulates breast development.
从孕中期开始，胎盘自身接管类固醇激素的产生。孕酮维持子宫内膜、抑制子宫收缩并促进胎盘发育。雌激素促进子宫生长并刺激乳房发育。
Relaxin, another placental hormone, softens the pubic symphysis and helps dilate the cervix in preparation for birth. In late pregnancy, oxytocin from the posterior pituitary triggers uterine contractions during labour.
胎盘释放的松弛素使耻骨联合软化，并帮助宫颈在分娩前扩张。妊娠晚期，垂体后叶释放的催产素在分娩过程中触发子宫收缩。
7. Diagnosis of Pregnancy | 妊娠诊断
The earliest reliable method is a pregnancy test that detects hCG in urine or blood. hCG levels rise rapidly after implantation, doubling every 2-3 days during the first weeks. Home urine tests can detect hCG about 14 days after conception.
最早可靠的方法是检测尿液或血液中的hCG。着床后hCG水平迅速升高，最初几周每2-3天翻倍。家用尿液检测可在受孕后约14天检出hCG。
Ultrasound scanning is used to confirm viability, estimate gestational age, and detect multiple pregnancies. The crown-rump length measured at 8-14 weeks provides an accurate estimate of gestation.
超声扫描用于确认胎儿存活、估算孕龄和检测多胎妊娠。在8-14周测量的头臀长可提供准确的孕龄估算。
Other tests include Doppler monitoring of fetal heart rate and blood screening for alpha-fetoprotein (AFP) to detect neural tube defects.
其他检测包括胎心率多普勒监测，以及通过血液筛查甲胎蛋白（AFP）来检测神经管缺陷。
8. Labour and Birth | 分娩
Labour is divided into three stages. In the first stage, uterine contractions progressively dilate the cervix to 10 cm. This stage may last 8-18 hours for a first birth.
分娩分为三个阶段。第一产程，子宫收缩使宫颈扩张至10厘米。初产妇此阶段可能持续8-18小时。
The second stage involves expulsive contractions and maternal pushing, delivering the baby through the vagina. The third stage delivers the placenta and membranes, often within 15-30 minutes after birth.
第二产程包括排挤性收缩和母亲用力，通过阴道娩出胎儿。第三产程在出生后15-30分钟内娩出胎盘和胎膜。
Oxytocin is often administered clinically to induce or strengthen contractions. Breastfeeding soon after birth stimulates natural oxytocin release, helping to deliver the placenta and reduce maternal haemorrhage.
临床上常使用催产素来引产或加强宫缩。出生后及时哺乳会刺激自然催产素释放，有助于胎盘娩出并减少产妇出血。
9. Nutrition and Health in Pregnancy | 孕期营养与健康
Adequate nutrition is essential for fetal development. Folic acid (400 µg daily before and during early pregnancy) reduces the risk of neural tube defects. Iron and calcium requirements increase during pregnancy.
充足营养对胎儿发育至关重要。叶酸（孕前和孕早期每日400微克）可降低神经管缺陷风险。妊娠期铁和钙的需求增加。
Substances such as alcohol, nicotine, and certain drugs can cross the placenta and cause fetal alcohol syndrome, low birth weight, or developmental abnormalities. Pregnant women are advised to avoid raw seafood, unpasteurised dairy, and excessive caffeine.
酒精、尼古丁和某些药物可穿过胎盘，导致胎儿酒精综合征、低出生体重或发育异常。建议孕妇避免生海鲜、未经巴氏消毒的乳制品和过量咖啡因。
- Weight gain should be gradual, not excessive.
- 体重增加应平缓，不宜过多。
- Regular moderate exercise is beneficial unless contraindicated.
- 定期适度运动有益，除非有禁忌。
10. Complications and Risk Factors | 并发症与风险因素
Ectopic pregnancy occurs when the embryo implants outside the uterus, usually in the oviduct. This is a medical emergency causing severe pain and bleeding. Gestational diabetes and pre-eclampsia are common complications that require monitoring.
异位妊娠是指胚胎在子宫外着床，通常在输卵管内。这是急症，会引起剧烈疼痛和出血。妊娠糖尿病和子痫前期是需要监测的常见并发症。
Pre-term birth (before 37 weeks) can occur due to cervical incompetence, infection, or maternal illness. Babies born before 30 weeks are at higher risk for respiratory distress syndrome because their lungs lack surfactant.
早产（孕37周前）可能由宫颈机能不全、感染或母体疾病引起。30周前出生的婴儿因肺部缺乏表面活性物质，患呼吸窘迫综合征的风险更高。
Multiple pregnancies (twins or triplets) increase the risk of premature birth and low birth weight. Regular antenatal check-ups help detect these issues early.
多胎妊娠（双胞胎或三胞胎）会增加早产和低出生体重的风险。定期产前检查有助于及早发现这些问题。
11. Postnatal Adjustments | 产后适应
At birth, the newborn must make rapid physiological changes. The first breath inflates the lungs, causing a drop in pulmonary vascular resistance. The foramen ovale closes, and the ductus arteriosus constricts, shifting blood flow to the lungs.
出生时，新生儿必须迅速做出生理调整。第一次呼吸使肺膨胀，肺血管阻力下降。卵圆孔关闭，动脉导管收缩，血液流动转向肺部。
The umbilical cord is clamped and cut, separating the neonate from the placenta. The remnant of the umbilical vein becomes the ligamentum teres hepatis in the liver.
脐带被夹紧并剪断，使新生儿与胎盘分离。脐静脉残余部分在肝脏中成为肝圆韧带。
Immediately after birth, the baby receives antibodies through colostrum, the first breast milk, providing passive immunity. Vitamin K is often given to prevent bleeding because the newborn's gut lacks bacteria to synthesise it.
出生后，婴儿通过初乳（第一种母乳）获得抗体，产生被动免疫。通常还会给予维生素K以预防出血，因为新生儿肠道缺乏合成维生素K的细菌。
12. Summary | 总结
The 280-day pregnancy is a dynamic process involving fertilisation, implantation, embryonic and fetal development, maternal adaptation, hormonal control, and eventual birth. Understanding each stage helps explain common pregnancy-related conditions and the importance of antenatal care.
280天妊娠是一个动态过程，涉及受精、着床、胚胎和胎儿发育、母体适应、激素调控及最终分娩。理解每个阶段有助于解释常见的妊娠相关状况以及产前保健的重要性。
Key facts to remember for IGCSE Biology: the placenta acts as a selective barrier, hCG maintains early pregnancy, the fetal circulatory system includes two shunts, and labour is triggered by oxytocin. These concepts are frequently tested in examinations.
IGCSE生物学需要记住的关键事实：胎盘作为选择性屏障，hCG维持早期妊娠，胎儿循环系统包含两个分流，分娩由催产素触发。这些概念在考试中常被考查。
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